Allow plugins to run before/after a rebar command.

This patch makes a small change in rebar_core that checks the list
of valid plugins to see if any of them export a pre/post processing
function for the current command. This logic is applied only to the
plugins and allows plugin authors to hook into rebar's execution by
using a naming convention that matches the one used for scripting hooks.

Example:

```erlang
-module(my_rebar_plugin).
-export([pre_compile/2]).
pre_compile(Config, AppFile) ->
    rebar_log:log(debug, "PRECOMPILE: ~p:~p~n",
                  [AppFile, Config]),
    ok.
```

0 files changed, 0 insertions, 0 deletions