This patch updates rebar_core to look for missing plugins (i.e. those
that aren't found on the code path at runtime) in a configurable
plugin directory, and dynamically compile and load them at runtime.
By default, the directory "plugins" is searched, although this can be
overriden by setting the plugin_dir in your rebar.config.
This change makes it possible to assign pre/post scripts to all
rebar commands. This allows users fine grained control over
when scripts and/or shell commands should be executed, where such
extensions are absolutely needed.
Several examples have been added to the rebar.config.sample file.