Add linters for WP projects

[JakubSzajna]

Should we add some linters for wordpress?

https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards

We can use those rulesets and embed phpcs binary in chisel.

PHPMD and PHPCPD could also be useful, but I haven't tested how it will work with WP. But we could scan only wp-content/themes directory.