Add the Mercury language to linguist

Mercury is a logical/functional language.  It was first developed at The University of Melbourne and is now almost 20 years old.  Projects on github include:

https://github.com/Mercury-Language/mercury
https://github.com/PaulBone/protobuf-mercury
https://github.com/PaulBone/pbone_thesis
https://github.com/wangp/bower
https://github.com/juliensf/mercury-csv
https://github.com/juliensf/mercury-misc
https://github.com/juliensf/mercury-json
https://github.com/petdr/venus

Thanks.

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.