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.
```

src/rebar_core.erl

@@ -148,8 +148,16 @@ process_dir(Dir, ParentConfig, Command, DirSet) ->
                     %% in preprocess.
                     {ok, PluginModules} = plugin_modules(Config),
 
+                    %% Execute any before_command plugins on this directory
+                    execute_pre(Command, PluginModules,
+                                Config, ModuleSetFile),
+
                     %% Execute the current command on this directory
                     execute(Command, Modules ++ PluginModules,
+                            Config, ModuleSetFile),
+
+                    %% Execute any after_command plugins on this directory
+                    execute_post(Command, PluginModules,
                                  Config, ModuleSetFile)
             end,
 
@@ -215,6 +223,17 @@ is_dir_type(rel_dir, Dir) ->
 is_dir_type(_, _) ->
     false.
 
+execute_pre(Command, Modules, Config, ModuleFile) ->
+    execute_plugin_hook("pre_", Command, Modules,
+                        Config, ModuleFile).
+
+execute_post(Command, Modules, Config, ModuleFile) ->
+    execute_plugin_hook("post_", Command, Modules,
+                        Config, ModuleFile).
+
+execute_plugin_hook(Hook, Command, Modules, Config, ModuleFile) ->
+    HookFunction = list_to_atom(Hook ++ atom_to_list(Command)),
+    execute(HookFunction, Modules, Config, ModuleFile).
 
 %%
 %% Execute a command across all applicable modules