RFC libquran: Plugin management

["Mohammad DAMT" <mdamt at gnome dot org>]

The plugins will read into memory when needed (on Linux it will use
dlopen syscall) and will be discarded when the audio files are closed.

All plugins shall be resided in the directory known to the library
(the default Linux installation is /usr/lib/libquran/). Each plugin
shall implement these interfaces (exact arguments are coming soon):

quran_plugin_init ()
quran_audio_play ()
quran_audio_stop ()
quran_audio_get_info ()
quran_plugin_destroy ()