Add the Mercury language to linguist

lib/linguist/languages.yml:
    Add the declaration for the language.

samples/Mercury:
    Add samples for the classifier as Mercury shares it's filename extension
    with several other languages.
MATLAB uses .m files not .matlab.
Reverted to the .matlab extension and added .m as another extension.
Added .h as an Objective-C file type
Changed the order of the Objective-C files.
Trimmed white space.
Added Stata extensions.
Added .ado and .sthlp samples for Stata.

The author of this message, mwhite-IPA, is the source of these samples.
Added .mata sample for Stata.

Source: http://www.stata.com/help.cgi?m1_first
Added .do sample for Stata.

Source: http://www.stata.com/help.cgi?regress
Added .ihlp sample for Stata.

The author of this message, mwhite-IPA, is the source of this sample.
Added .doh and .matah samples for Stata.

Source: http://www.stata.com/help.cgi?include
Added Text only lexer for Stata.
Rebol extension changes to languages.yml

Historically Rebol used the .r extension which unfortunately clashes with the R Stats programming language :(

For example even the Rebol interpreter repo says it uses 9.7% R instead of Rebol! - https://github.com/rebol/rebol

So 3 changes here…

1) .reb is now the primary (and official) extension - http://www.rebol.com/article/0540.html

2) .rebol moved to list of extensions (some code does use it)

3) .r added back.  NB. The majority of Rebol code on Rebol uses this (followed by .r2 & .r3).  .r was present in languages.yml previously but was removed for some reason? (looks like here - https://github.com/github/linguist/commit/5a5d33499942d2a294ffec593539a9095f105882)
Sample file for .reb Rebol extension
Sample file for .rebol Rebol extension
Sample file for .r2 Rebol extension
Sample file for .r3 Rebol extension
Sample file for .r Rebol extension
C++ .inl file extension