toolcontext: new option -w to enable/disable warning

The `-w` option takes the name of a warning (displayed at the end of message, between parentheses) to activate it; and "no-{name}" to disable it. It has precedence over -q and -W.

To show only missing-doc warnings in standard
~~~sh
$ nitg -q -w missing-doc standard
~~~

To show all warnings and advices, except missing-doc
~~~sh
$ nitg -W -w no-missing-doc standard
~~~

To show standard warnings except useless-type-test, but not advice except missing-doc
~~~sh
$ nitg -w missing-doc -w no-useless-type-test standard
~~~