Auto format generated templates

[leandrocp]

While working on https://github.com/phoenixframework/phoenix/pull/4820 I've realized that manually formatting templates with a default config won't work well because the user's project may have a different config for line length, generating a file that won't be formatted in that project context, forcing the user to run mix format to fix it anyways. Besides that a project may have custom plugins that is expected to modify those files.

So why not format on demand? The test shows how it could work. WDYT?