Add an ability to configurate list of files to convert to be used by javascript

[antonkomarev]

I've added configuration file for your package.

Right now there could be defined list of language files which should be included in javascript output.

Checks are done via separate method for easier extending functionality in future:

• define excludes via console command
• add ability to include only exact messages in language files, not the whole files:

'messages' => [
    'passwords.reset',
    'pagination.next',
    'pagination.previous',
],

2 more cases covered in unit tests, code cleaned up and formatted in PSR-2 (because it's standard since Laravel 5.1).

[rmariuzzo]

Thank you very much @a-komarev!

[rmariuzzo]

@vluzrmos I'm considering on adding @a-komarev as a contributor. What do you think? I'm basing my decision because on his contributions on this repo, and Lang.js.