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.

lib/linguist/languages.yml

@@ -1089,6 +1089,14 @@ MediaWiki:
   wrap: true
   primary_extension: .mediawiki
 
+Mercury:
+  type: programming
+  # This is the background colour on the web page.
+  color: "#abcdef"
+  primary_extension: .m
+  # Mercury's syntax is not prolog syntax, but they do share the lexer
+  lexer: Prolog
+
 MiniD: # Legacy
   searchable: false
   primary_extension: .minid # Dummy extension