gitignore all files in www/default except for index.php

Rather than explicitly listing the files we don't want added to the repo, ignore all of them and explicitly allow index.php only.

This opens things up a bit more for custom dashboards
Allow for a custom dashboard to replace the vvv.dev default

Uses a file_exists check to load `dashboard-custom.php` if it's available so that a custom dashboard can be provided for VVV

Closes #185