awful: add generic completion wrapper for awful.prompt.run()

awful: add generic completion wrapper for awful.prompt.run()

[completion.lua.in]:
  * add generic() function for completion

[prompt.lua.in]
  * add completion_run() function as a run() wrapper with completion

kw_feeder sould return a key = value dictionnary. Completion is run against
key. Once completion is done, exe_callback(value, key) is executed.
That's the major differences between awful.prompt.run() and
 awful.prompt.completion_run

Notice: This is not thread safe but as longs as only one keygrabber could be
run who cares?

Signed-off-by: Sébastien Gross <seb-awesome@chezwam.org>
Signed-off-by: Julien Danjou <julien@danjou.info>