Support for some repeated tags in mpd widget type

Fields Artists and Genres has been added
for the mpd widget type. They are concatenated
Artist and Genre tags that are repeated, respectively.

Addtion of AMD GPU widget type for Linux

Some typos in contrib docs have also been fixed.

Various bug fixes and improvements

Fixed:

• Escaping of % in helpers.format, which affects mpd widget ${Progress}
• Possible deadlock of when update widgets
• [contrib.openweather] New API compatibility, which requires an API key
• [gmail] Authentication documentation

Added:

• [mpd] Support for sending arbitrary commands
• [contrib.openweather] Various new return values

Add vicious.call_async and rework documentation

Fixed: vicious.call freezing awesome when used with asynchronous widget types

Added: vicious.call_async asynchronous analogous to vicious.call

Moved:

• Most of the documentation in READMEs to docs/
• Changes.md to CHANGELOG.rst
• CONTRIBUTING.md to ``CONTRIBUTING.rst`
• Meta helpers to tools/

Bring back sysfs path cache for hwmontemp widget type

v2.4.2

Nitpick whitespace and bump version

Fix regressions in v2.4.0

Fixed:

• [pkg] Fallback the number of lines before packages listing to 0.
  This fixes crashes on Arch, FreeBSD and Mandriva.
• [mdir] Remove trailing semicolon at the end of command.

Use Awesome async API and update copyright notices

IMPORTANT:

• volume now uses 🔉 and 🔈 instead of ♫ and ♩ to show mute state.
  This BREAKS backward compatibility if users substitute custom symbols
  from these default.

Added:

• notmuch_all, cpu_freebsd widget types.
• [cmus_all] Promote to widgets/.
• [wifiiw_linux] Expose BSSID.
• [wifi_linux] Expose frequency and transmission power.
• spawn as a fallback for awful.spawn in case Vicious is used as
  a stand-alone library. This wrapper, however, does NOT provide the facilities
  to asynchronously spawn new processes. It also lacks a few features such as
  parsing stderr and returning PID.
• helpers.setasyncall to avoid writing redundant workers for asynchronous
  widget types. Note that these workers are only needed in case Vicious is used
  as a stand-alone library.
• helpers.setcall for registering functions as widget types.
• headergen script for automatic generation of copyright notices.
• templates for the ease of adding new widget types.
• CONTRIBUTING.md which guide contributors through the steps
  of filing an issue or submitting a patch.

Fixed:

• Deprecate the use of io.popen in following widgets:
  • wifi_linux, wifiiw_linux, hwmontemp_linux, hddtemp_linux
  • bat_freebsd, mem_freebsd, net_freebsd, thermal_freebsd, uptime_freebsd,
    cpu_freebsd, cpufreq_freebsd, fanspeed_freebsd
  • bat_openbsd
  • volume, gmail, mdir, mpd, fs
• [mpd] Lua 5.3 compatibility (for real this time); also correct a typo
• [mbox] Update the deprecated string.gfind to string.gmatch
• [pkg,weather,contrib/btc] Allow function call without Awesome
• [pkg] Use more updated front-ends for Debian/Ubuntu (apt) and Fedora (dnf)
• [os] Splitted os_all into os_linux and os_bsd (and refactored to async)
• Tweak .luacheckrc to suit functional style and soft-limit text width to 80
• Update copyright headers for libraries and widget types

Removed:

• helpers.sysctl and helpers.sysctl_table were removed in favour of
  helpers.sysctl_async.

Fix Lua 5.3 compatibility

Ensure Lua 5.3 compatibility for mpd_all.

Also add battery widget type for OpenBSD and update battery state symbols for FreeBSD.

Some minor bugfixes and improvements

Features:

• Support stacked graphs
• [hwmontemp_linux] Provide name-based access to hwmon sensors via sysfs
• [mpd_all] Expose more informations and format time in [hh:]mm:ss

Fixes:

• Improve defaults and mechanism for data caching
• Escape XML entities in results by default
• [weather_all] Update NOAA link and use Awesome asynchronous API
• [mem_linux] Use MemAvailable to calculate free amount
• [mem_freebsd] Correct calculation and switch to swapinfo for swap
• [bat_freebsd] Add critical charging state
• [fs_all] Fix shell quoting of option arguments

Moreover, .luacheckrc was added and README.md was refomatted for the ease of development.