support updating of per app info by hooks

[tsloughter]

This patch allows for a hook to update the opts of an appinfo record, hopefully being enough for what @Licenser wants in #670

An example would be a provider that you define in provider_hooks to run before compile which adds to erl_opts:

do(State) ->
  AppInfo = rebar_state:current_app(State),
  ErlOpts = rebar_app_info:get(AppInfo, erl_opts, []),
  AppInfo1 = rebar_app_info:set(AppInfo, erl_opts, [{d, 'SOME_DEFINE'} | ErlOpts]),
  {ok, rebar_state:current_app(State, AppInfo1)}.

[ferd]

+1

[tsloughter]

@Licenser confirmed in irc that this looks great, so ok to merge now?