Improve flycheck-valid-checker-p

Also make sure that we have a symbol
Add a function to validate a next-checker cell
[Fix #266] Add next checkers to syntax checkers
Merge branch 'extend-syntax-checkers'

* extend-syntax-checkers:
  [Fix #266] Add next checkers to syntax checkers
  Add a function to validate a next-checker cell
  Improve flycheck-valid-checker-p