Allow users to skip requirements check(s)

[stork]

There are use cases, where cakephp/app requirements are not met on the target system, but user still wants the Oven to create CakePHP app locally. For example, missing php-intl extension (there is workaround - CakeDC/Intl plugin). Some checkbox(es) (checked by default) causing --ignore-platform-reqs parameter of composer call(s) should do the trick.

[stork]

WIP https://github.com/stork/oven/commits/feature/37