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/rot13_concise.m

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+% File: rot13_concise.m
+% Main authors: Warwick Harvey <wharvey@cs.monash.edu.au>
+%               Fergus Henderson <fjh@cs.mu.oz.au>
+%
+% rot13_concise:
+%
+% Program to read its input, apply the rot13 algorithm, and write it out
+% again.
+%
+% This version is more concise (but less efficient) than its companion,
+% rot13_verbose.
+%
+% Key features:
+% - is independent of character set (e.g. ASCII, EBCDIC)
+% - has proper error handling
+%
+
+:- module rot13_concise.
+
+:- interface.
+:- import_module io.
+
+:- pred main(state, state).
+:- mode main(di, uo) is det.
+
+:- implementation.
+:- import_module char, int, string.
+
+% The length of `alphabet' should be a multiple of `cycle'.
+alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".
+cycle = 26.
+
+rot_n(N, Char) = RotChar :-
+	char_to_string(Char, CharString),
+	( if sub_string_search(alphabet, CharString, Index) then
+		NewIndex = (Index + N) mod cycle + cycle * (Index // cycle),
+		index_det(alphabet, NewIndex, RotChar)
+	else
+		RotChar = Char
+	).
+
+rot13(Char) = rot_n(13, Char).
+
+main -->
+	read_char(Res),
+	( { Res = ok(Char) },
+		print(rot13(Char)),
+		main
+	; { Res = eof }
+	; { Res = error(ErrorCode) },
+		{ error_message(ErrorCode, ErrorMessage) },
+		stderr_stream(StdErr),
+		print(StdErr, "rot13: error reading input: "),
+		print(StdErr, ErrorMessage),
+		nl(StdErr)
+	).
+
+