Add gherkin-lint-php

[dkarlovi]

The tool is very new: https://github.com/dantleech/gherkin-lint-php

I don't know if there's plans for integration with Behat in some way, maybe @ciaranmcnulty knows more? Either way, it would complement current linters nicely.

[ciaranmcnulty]

The parser it's using is one day going to be also used by behat; I don't think we have plans to add a linter

[jakzal]

For anyone considering contributing, here's the guide: https://github.com/jakzal/toolbox/blob/master/CONTRIBUTING.md