symfony-console arguments handling + WordPress Timber bridge

[drzraf]

• This commit uses symfony-console for manage arguments specific to Twig-Gettext-Extractor which used to be broken.
• The $input is also passed to the extractor for possible futur use (eg: handling --debug)
• Better Twig loaded. Twig Environment use a ChainLoader which is initialized according to file/paths passed to the command line.
• It's possible to add a template directory the Twig-way and compile for one (or multiple templates).
• The list of Twig templates to compile can now be passed through stdin, using --files stdin
• Twig_SimpleFilter is fixed. true is not an accepted value anymore. Empty value callback avoid the cryptic Twig error: Function 1() does not exist
• A --require flag is added to add custom PHP code before compilation.
• Using the above, a brigde with WordPress+Timber is added. It loads Timber Twig extensions and add a couple of WordPress specific parameters to xgettext.

[umpirsky]

Please update with latest master, travis fails now.

[umpirsky]

@drzraf What is the status of the PR? Looks like we only need to update README to be able to setup Poedit.

[drzraf]

I updated the command in README.md, but not the text that appears inside the screenshot (new screenshot is needed). Could you?

[umpirsky]

Sure, let's fix tests first.