Update wp-cli on during provisioning

No sense in letting wp-cli get stale on us. Using rebase here to throw a smarter error when there are unstaged changes in the wp-cli working directory. Not sure if that's best, but it'll do for now.

Closes #82
Set WP_DEBUG with wp-cli extra-php parameter

Now that we're updating wp-cli on provision, we have access to the new extra-php parameter in wp-cli. This allows us to add PHP code to the wp-config.php file as it is created by wp-cli through the use of here-docs.

Here-docs are ugly and screw with the indentation of the provision.sh file, but that's cool. It might be worth piping in PHP from a file if possible and if we ever get to the point where we need something more complicated set.

Closes #81