Dynamic configuration

With rebar.config, reltool.config, *.app.src, and overlay vars files
you can make use of dynamic configuration based on file:script/2.

If a <name>.script exists in the same directory as the original file (in the case of
rebar.config that would be rebar.config.script, the script file will be
evaluated and the result used as configuration. This also works for custom
rebar.config files. That means rebar -C special_config will check for
existence of special_config.script.

For convenience two bindings (variables) are available in a script during evaluation:

1. CONFIG - result of file:consult/1 if the script file being evaluated also exists
   without the .script extension. Otherwise, [].

2. SCRIPT - filename of the script being evaluated

In all cases, the data returned by the script, that is the last thing evaluated in the file,
must be data in the same format as the original non-script file. For example, if I have
rebar.config.script that script must return rebar configuration data, if it is an
<app-name>.app.src.script it must return data in the Application Metadata Format.

Simple Example

If you are building fairly complex systems, going to github to fetch deps each time will
slow down your development cycle. Serving dependencies locally may be a much faster route,
but you don't want to modify the rebar.config and suffer merge conflicts as a result.

The following rebar.config.script file can be kept centrally, and linked into your
application directory:

case os:getenv("REBAR_DEPS") of
    false -> CONFIG; % env var not defined
    []    -> CONFIG; % env var set to empty string
    Dir ->
	lists:keystore(deps_dir, 1, CONFIG, {deps_dir, Dir})
end.

Whenever you want to build 'properly' (which you should, regularly), simply call
unset REBAR_DEPS (or equivalent), and perform a clean build.

Note that file:script/2 differs from file:consult/1 in that only the result of the
last expression is returned. You must therefore take care to return a list of config items.
Before that, you may do any form of IO (including network), checking OS environment variables,
reading files (perhaps calling file:script/2 on other files) or writing files. You can
basically use all OTP libs. As in file:eval/2, each expression is terminated with a full stop.