Add SSH config lexer (#1543)

This commit adds a lexer for SSH configuration files.

Co-authored-by: Michael Camilleri <mike@inqk.net>

lib/rouge/demos/ssh

@@ -0,0 +1,4 @@
+Host example
+    Hostname example.com
+    User user
+    Port 1234

lib/rouge/lexers/ssh.rb

@@ -0,0 +1,33 @@
+# -*- coding: utf-8 -*- #
+# frozen_string_literal: true
+
+module Rouge
+  module Lexers
+    class SSH < RegexLexer
+      tag 'ssh'
+
+      title "SSH Config File"
+      desc 'A lexer for SSH configuration files'
+      filenames 'ssh_config'
+
+      state :root do
+        rule %r/[a-z0-9]+/i, Keyword, :statement
+        mixin :base
+      end
+
+      state :statement do
+        rule %r/\n/, Text, :pop!
+        rule %r/(?:yes|no|confirm|ask|always|auto|none|force)\b/, Name::Constant
+
+        rule %r/\d+/, Num
+        rule %r/[^#\s;{}$\\]+/, Text
+        mixin :base
+      end
+
+      state :base do
+        rule %r/\s+/, Text
+        rule %r/#.*/, Comment::Single
+      end
+    end
+  end
+end

spec/lexers/ssh_spec.rb

@@ -0,0 +1,37 @@
+# -*- coding: utf-8 -*- #
+# frozen_string_literal: true
+
+describe Rouge::Lexers::SSH do
+  let(:subject) { Rouge::Lexers::SSH.new }
+
+  describe 'lexing' do
+    include Support::Lexing
+
+    it 'lexes a specification' do
+      example = 'Host example
+                 Hostname www.example.com
+                 Port 1234
+                 Tunnel no'
+
+      assert_tokens_equal example, ["Keyword",        "Host"],
+                                   ["Text",           " example\n                 "],
+                                   ["Keyword",        "Hostname"],
+                                   ["Text",           " www.example.com\n                 "],
+                                   ["Keyword",        "Port"],
+                                   ["Text",           " "],
+                                   ["Literal.Number", "1234"],
+                                   ["Text",           "\n                 "],
+                                   ["Keyword",        "Tunnel"],
+                                   ["Text",           " "],
+                                   ["Name.Constant",  "no"]
+    end
+  end
+
+  describe 'guessing' do
+    include Support::Guessing
+
+    it 'guesses by filename' do
+      assert_guess :filename => '/etc/ssh_config'
+    end
+  end
+end

spec/visual/samples/ssh

@@ -0,0 +1,15 @@
+# This is the ssh client system-wide configuration file.  See
+# ssh_config(5) for more information.  This file provides defaults for
+# users, and the values can be changed in per-user configuration files
+# or on the command line.
+
+Host *
+    Port 22 # default port
+    Tunnel no
+    GSSAPIAuthentication yes
+    GSSAPIDelegateCredentials no
+
+Host example
+    Hostname www.example.com
+    User user
+    Port 1234