Adding basic Lua scripting basics to mutt

Adding basic Lua scripting basics to mutt

* adding two commands:
 - lua: to parse a line of lua code
 - lua-source: to load and parse a lua file
* binding two mutt functions in lua:
 - mutt_message and
 - mutt_error

cf #153

Signed-off-by: Guyzmo <guyzmo+github+pub@m0g.net>