Add --adapter flag

[moogle19]

Adds a --adapter flag to mix phx.new to choose the web adapter for a new application (allows cowboy or bandit, defaults to cowboy).

I am not quite happy with having the versions in the generator.ex, but not sure if using >= 0.0.0 (like for the ecto_adapters) will lead to problems in the future.

[Gazler]

I'm not sure if we're ready to merge this yet, but it's worth updating the following files too:

https://github.com/phoenixframework/phoenix/blob/master/installer/templates/phx_umbrella/apps/app_name_web/config/config.exs
https://github.com/phoenixframework/phoenix/blob/master/installer/templates/phx_umbrella/apps/app_name_web/mix.exs

[mtrudel]

I was planning on submitting a PR for this after 1.7 was out, on the reasoning that we should probably let bandit support bake in for a bit in an 'unofficial' capacity before having Phoenix put any sort of 'official' stamp on it (There's also some documentation changes & other minor stuff that's in a similar boat). This feels like a 1.7.x type thing, lest Bandit support get a bad name if anything serious comes up during the inevitable teething process.

When the time does come, this PR looks great though!

[mtrudel]

There's also test coverage to be added here - feel free to take at will from the (mostly done, but not heavily tested) copy of the work I've done on this! https://github.com/mtrudel/phoenix/tree/bandit_generator

[mtrudel]

Looks great! Maybe we can revisit inclusion with 1.7.x (for some small x)?

[mtrudel]

This continues to look good to me; I'll leave it at the discretion of @josevalim et al about timing, but from my perspective this is ready to go anytime.

[chrismccord]

❤️❤️❤️🐥🔥

[mtrudel]

Woooo!