Add flycheck options

[Fuco1]

Add options to set the strictness level (defaults to 0) and config file (defaults to none).

These can be used on per-project basis with a .dir-locals.el for example

((nil
  (flycheck-phpstan-level . "7"))

[zonuexe]

@Fuco1 Please merge either #3 or #4 and close the rest. Thank you!