Add ability to customize options injected into LeagueCommonMarkConverterFactory

[Pierstoval]

This way, adding custom options can be done by overriding the service in DI's config:

services:
  Twig\Extra\TwigExtraBundle\LeagueCommonMarkConverterFactory:
    arguments:
      - !tagged_iterator { tag: twig.markdown.league_extension }
      - # custom config array for League\CommonMark\CommonMarkConverter's constructor