Using nimscript for configuration

The [nimscript docs](* https://nim-lang.org/docs/nimscript.html) describe the nimscript API, but I needed some help on how to use it. (Most of the following applies to nimble too, but let's concentrate on nimscript for now.)

Example

Suppose you have a scrDir like this:

src/
    foo.nim
    foo.nim.cfg
    foo.nims
    nim.cfg
    zzz.nims

Suppose foo.nims looks like this:

echo "In foo.nims"

include "zzz.nims"

task mybuild, "My build task":
  echo "In mybuild"
  switch("verbosity", "2")
  setCommand "c"

task newtask, "My new task":
  echo "In newtask"
  setCommand "dump"

And zzz.nims looks like this:

echo "In zzz.nims"
echo gorge("git rev-parse HEAD")

You could run your nimscript like this: nim e foo.nims, but that will not make your tasks available.

Instead, run nim on foo.nim like this:

nim newtask foo
nim mybuild -d:debug foo

All the standard .cfg files are read before your nimscript. You can use include to avoid repeating code for each tool-specific .nims file. That's not quite as convenient as a global nim.cfg file, but it's more flexible.