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.

samples/Mercury/hello.m

@@ -0,0 +1,14 @@
+% "Hello World" in Mercury.
+
+% This source file is hereby placed in the public domain.  -fjh (the author).
+
+:- module hello.
+:- interface.
+:- import_module io.
+
+:- pred main(io::di, io::uo) is det.
+
+:- implementation.
+
+main(!IO) :-
+	io.write_string("Hello, world\n", !IO).