Add detection for the M programming language (aka MUMPS).

This add detection for the M programming language (see https://en.wikipedia.org/wiki/MUMPS or the ISO/IEC standard of the language http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=29268).

I have successfully tested this using `bundle exec rake test`.

I have also called `bundle exec linguist` on all the projects hosted on GitHub which I know have M files (not .m files) in them, and they would now all be correctly categorized as M instead of Objective-C or Matlab :

    90%  M
    6%   C
    3%   Objective-C
    1%   Matlab
    0%   Shell
    0%   Assembly
    0%   Python

I have also called `bundle exec linguist` on all the popular Objective-C projects found on GitHub (https://github.com/languages/Objective-C) with the following result :

    62%  Objective-C
    11%  C
    10%  JavaScript
    9%   Java
    5%   Python
    3%   C++
    0%   PHP
    0%   Shell
    0%   C#
    0%   Ruby
    0%   Perl
    0%   Lua

This patch clearly doesn't impact Objective-C detection based on defaulting to Objective-C for .m files.

This patch won't affect Matlab because Matlab's regex comes before the ones this patch is adding.