Support multiple PHP versions

[philipnewcomer]

Support multiple versions of PHP, switchable via a cookie like Xdebug is currently toggled.

[AaronHolbrook]

Also a query arg would be nice for this

[AaronHolbrook]

Would also be nice to not install a ton of php versions by default, but instead only if user directs dockerbox to do so (goes back to the idea of somehow allowing customization by user)

[philipnewcomer]

Added support for multiple versions of PHP, switchable via a php cookie and a query arg, similar to how Xdebug works already. The current Xdebug toggle/query arg plays nicely with the PHP toggle/query arg.

I decided to officially support in Pilothouse only the versions of PHP currently supported by the PHP maintainers themselves, 5.6, 7.0, and 7.1 currently. While I wouldn't rule out older versions of PHP, there are no official Docker images for unsupported PHP versions, so they will be much more difficult to support in Pilothouse.

Documentation
Changing the PHP version for web requests:
https://github.com/Pilothouse-App/pilothouse/wiki/Changing-a-site's-PHP-version
Changing the PHP version for WP-CLI commands:
https://github.com/Pilothouse-App/pilothouse/wiki/Using-WP-CLI