Add option --no-color to suppress colorization of output

By default, if output is a TTY, dotbot will colorize the output. This
patch adds the option to pass `--no-color` to dotbot to have it suppress
this colorization.

dotbot/cli.py

@@ -28,6 +28,8 @@ def add_options(parser):
         action='store_true', help='disable built-in plugins')
     parser.add_argument('--plugin-dir', action='append', dest='plugin_dirs', default=[],
         metavar='PLUGIN_DIR', help='load all plugins in PLUGIN_DIR')
+    parser.add_argument('--no-color', dest='no_color', action='store_true',
+        help='disable color output')
     parser.add_argument('--version', action='store_true',
         help='show program\'s version number and exit')
 
@@ -50,6 +52,8 @@ def main():
             log.set_level(Level.INFO)
         if options.verbose:
             log.set_level(Level.DEBUG)
+        if options.no_color:
+            log.use_color(False)
         plugin_directories = list(options.plugin_dirs)
         if not options.disable_built_in_plugins:
             from .plugins import Clean, Link, Shell

dotbot/messenger/messenger.py

@@ -7,10 +7,14 @@ from .level import Level
 class Messenger(with_metaclass(Singleton, object)):
     def __init__(self, level = Level.LOWINFO):
         self.set_level(level)
+        self.use_color(True)
 
     def set_level(self, level):
         self._level = level
 
+    def use_color(self, yesno):
+        self._use_color = yesno
+
     def log(self, level, message):
         if (level >= self._level):
             print('%s%s%s' % (self._color(level), message, self._reset()))
@@ -30,11 +34,14 @@ class Messenger(with_metaclass(Singleton, object)):
     def error(self, message):
         self.log(Level.ERROR, message)
 
+    def _should_use_color(self):
+        return self._use_color and sys.stdout.isatty()
+
     def _color(self, level):
         '''
         Get a color (terminal escape sequence) according to a level.
         '''
-        if not sys.stdout.isatty():
+        if not self._should_use_color():
             return ''
         elif level < Level.DEBUG:
             return ''
@@ -53,7 +60,7 @@ class Messenger(with_metaclass(Singleton, object)):
         '''
         Get a reset color (terminal escape sequence).
         '''
-        if not sys.stdout.isatty():
+        if not self._should_use_color():
             return ''
         else:
             return Color.RESET