Configuration management

The configuration files are now contained into a config subdirectory, so that it is easier to "move" or backup all the configuration files.
Moreover, the configuration files changed their name to (i) be not hidden and (ii) have a shorter name (e.g., 10.conf to refer to PostgreSQL 10.x configuration).
The special subocommand config migrate does the upgrade of your existing configuration from previous versions of pgenv.