Add Brainfuck lexer (#1037)

This adds support for the Brainfuck language.

lib/rouge/demos/brainfuck

@@ -0,0 +1,5 @@
+[
+	This is a sample
+	comment.
+]
+.[>+<-]>,

lib/rouge/lexers/brainfuck.rb

@@ -0,0 +1,53 @@
+# -*- coding: utf-8 -*- #
+# frozen_string_literal: true
+
+module Rouge
+  module Lexers
+    class Brainfuck < RegexLexer
+      tag 'brainfuck'
+      filenames '*.b', '*.bf'
+      mimetypes 'text/x-brainfuck'
+
+      title "Brainfuck"
+      desc "The Brainfuck programming language"
+
+      start { push :bol }
+
+      state :bol do
+        rule /\s+/m, Text
+        rule /\[/, Comment::Multiline, :comment_multi
+        rule(//) { pop! }
+      end
+
+      state :root do
+        rule /\]/, Error
+        rule /\[/, Punctuation, :loop
+
+        mixin :comment_single
+        mixin :commands
+      end
+
+      state :comment_multi do
+        rule /\[/, Comment::Multiline, :comment_multi
+        rule /\]/, Comment::Multiline, :pop!
+        rule /[^\[\]]+?/m, Comment::Multiline
+      end
+
+      state :comment_single do
+        rule /[^><+\-.,\[\]]+/, Comment::Single
+      end
+
+      state :loop do
+        rule /\[/, Punctuation, :loop
+        rule /\]/, Punctuation, :pop!
+        mixin :comment_single
+        mixin :commands
+      end
+
+      state :commands do
+        rule /[><]+/, Name::Builtin
+        rule /[+\-.,]+/, Name::Function
+      end
+    end
+  end
+end

spec/lexers/brainfuck_spec.rb

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+# -*- coding: utf-8 -*- #
+# frozen_string_literal: true
+
+describe Rouge::Lexers::Brainfuck do
+  let(:subject) { Rouge::Lexers::Brainfuck.new }
+
+  describe 'guessing' do
+    include Support::Guessing
+
+    it 'guesses by filename' do
+      assert_guess :filename => 'foo.b'
+      assert_guess :filename => 'FOO.bf', :source => 'foo'
+    end
+
+    it 'guesses by mimetype' do
+      assert_guess :mimetype => 'text/x-brainfuck'
+    end
+  end
+
+  describe 'lexing' do
+    include Support::Lexing
+
+    it 'recognizes one-line comments not followed by a newline (#796)' do
+      assert_tokens_equal 'comment', ['Comment.Single', 'comment']
+    end
+  end
+end

spec/visual/samples/brainfuck

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+[
+This program will print "Hello world!"
+]
+++++++++++
+[
+   >+++++++>++++++++++>+++>+<<<<-
+]
+>++. Initial loop (it will be printed an H)
+>+. e
++++++++. l
+. l
++++. o
+>++. space
+<<+++++++++++++++. W
+>. o
++++. r
+------. l
+--------. d
+>+. !
+>.