wp-now: Make it possible to use WP-CLI

[danielbachhuber]

Once WordPress/wordpress-playground#242 is done, we should add some convenient shorthand for WP-CLI.

Done is:

• wp-now cli executes WP-CLI.
• If WP-CLI hasn't been downloaded yet, wp-now downloads it to its magical hidden folder.

[danielbachhuber]

Once WP-CLI is available, we can use it to handle downloading WordPress too.

[adamziel]

• Related: Add terminal to playground site wordpress-playground#161
• Full wp-cli support may require building PHP with pthreads, but we may be able to get very far without it.

[danielbachhuber]

Another angle on this that's come up: what if I want to run my existing wp executable against a wp-now site (i.e. the user already has PHP and WP-CLI installed on their system)?

It might not actually be too difficult — we may just need to tell WP-CLI where to find the WordPress files, SQLite database, and project files. At the moment however, the knowledge of this only exists in the virtualized filesystem.