Add a script to help switch theme for local development

When working locally, it's useful to be able to switch between
themes quickly, which essentially involves:

  - Updating the general.yml symlink to point to a theme-specific one

  - Updating the public/alavetelitheme symlink

  - Making sure that the theme exists as vendor/plugins/<theme-name>

This script lets you switch between themes kept in a directory
which is by default called 'alaveteli-themes' at the same level
as your Alaveteli git repository, or can be overriden by the
environment variable ALAVETELI_THEMES_DIR.