Introduce a helper method for launching a new WP-CLI process using the same context as the current process.
Now that we have a method to launch new WP-CLI processes, use it to flush rewrite rules after setting the structure.

Fixes #814
These arguments should be optional
Call the PHP binary directly, and ensure all arguments are escaped
rename launch_wpcli() to launch_self() and make it work from inside a Phar file
clean up rewrite.feature
Merge pull request #882 from wp-cli/fix-wp-rewrite-structure

`wp rewrite structure` fails to properly set tag / category rewrite rules (2)