Added basic support for flycheck.
Reenable ignore-errors.
Add custom checker for checking CSS/JS files with phpcs.
Check if our checker can be used before enforcing it.

The buffer could be in some derived mode that flycheck wont activate
the checker for, which results in errors if we try to enforce it.
Fixed documentation.

Don't pretend to be flymake-phpcs.
Mark `drupal/flymake-phpcs-standard' as obsolete.

And as an alias for `drupal/phpcs-standard'.
Merge branch 'develop' into pr/35