Add an option to add PHP Parallel Lint

[raphaelstolt]

Using the option to be e.g. --parallel-lint or simply --lint would add the PHP Parallel Lint package as a development dependency, add related Composer scripts e.g. lint and lint-changes, and add it (i.e. lint-changes) to the Travis CI build configuration.

What's your opinion on this?

[jonathantorres]

Seems like an interesting project. Go for it, as long as it's optional. 👍