[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.

configuration/cdist.cfg.skeleton

@@ -13,6 +13,12 @@
 #     Specify cache path pattern.
 # cache_path_pattern = %h
 #
+# colored_output
+#     Use a colored output for different log levels.
+#     It can be a boolean or 'auto' (default) which enables this feature if
+#     stdout is a tty and disables it otherwise.
+# colored_output = auto
+#
 # conf_dir
 #     List of configuration directories separated with the character conventionally
 #     used by the operating system to separate search path components (as in PATH),