Add support for the --add-comments xgettext parameter

When using the I18n extension in Twig I miss the ability to write comments in the line before the `gettext` call in order to fill the "notes for translator" in the `.po` files. I think this is very useful to put the translator in the right context to make a better translation

I create a new reserved word `notes` so a trans expression will look like this:

````
{% trans %}
    There is one thing you have to check!
{% plural alerts %}
    There are {{ alerts }} things you have to check!
{% notes %}
    This refers to a pending actions that the user have to do and will be shown in the home page
{% endtrans %}
````

In poedit you have to add to the parser the parameter like this if you want to read those notes

`xgettext --add-comments=notes`


@fabpot this is the very first time I look into the Twig internals and I don't know if I made a good code, but there are just a few lines to review
The poedit parser (really is the gettext parser) is able to add the comment after any gettext call, so this is the main reason to make this pull request... 

Maybe you're thinking in other way to do this?