[UX] Add option to enable LogLevel-based coloured output.

This makes it easier for new and experienced users to run cdist with higher
verbosity levels, both to know that things are working as expected and to debug
issues.

Documentation has been modified accordingly and default behaviour is not
changed.

cdist/emulator.py

@@ -129,6 +129,9 @@ class Emulator(object):
             # if invalid __cdist_log_level value
             logging.root.setLevel(logging.WARNING)
 
+        colored_log = self.env.get('__cdist_colored_log', 'False')
+        cdist.log.ColorFormatter.USE_COLORS = colored_log == 'True'
+
         self.log = logging.getLogger(self.target_host[0])
 
     def commandline(self):