[2.x] Further API changes

[ryangjchandler]

• Removed $environment parameter from Phiki constructor.
  • This simplifies the internals a little and also makes for a cleaner API. I don't even imagine tonnes of people are using this as you'd lose support for a lot of default grammars and themes.
  • This was also designed as an internal abstraction for easier object sharing.
• Renamed some methods added in a previous release.
• Moved CommonMark related things into Phiki\Adapters\CommonMark namespace.
• Removed getGrammarRepository() and getThemeRepository() methods.
  • These are now just public properties on the environment object instead which you can get via $phiki->environment().