Restore the use of `--prompt` as a runtime argument.

The argument is unique of it's kind, and there isn't a great way of doing this otherwise.
Use new public methods of `\WP_CLI\Runner` to set the `url` if it was passed as an assoc arg by prompting.
Merge branch 'master' into fix-global-prompt-805
Introduce `\WP_CLI::set_url()`, which will set the URL context for the execution.
Use the new `set_url()` method introduced in ab45bfd
Move these methods to where they should be
Test install via prompting
move parse_url() to utils.php, since it's a standalone utility
Merge pull request #805 from wp-cli/fix-global-prompt-805

Prompting doesn't properly set config values