[TwigBundle] Add command to list twig functions, filters, globals and tests

Sample output:

```
$ app/console twig:doc date
Functions
  date(date = null, timezone = null)

Filters
  date(format = null, timezone = null)
  date_modify(modifier)
```

JSON output also available, could be helpful for IDEs.

Possible improvement would be to read the first line of the docblock of each method/function to show a bit more info.

Feedback welcome on the command's name. Not really convinced but too late to think of something better.

Credits to @gagarine for the great idea