Only allow alphanumeric characters for site name

[jancbeck]

Silly me wanted to use the domain name of a site and copied "example.com" from Chromes address bar, which was pasted as "http://example.com/" in vv create. This produced a couple errors during the setup script and vv delete couldn't clean up the mess either.
Would be nice to have a basic sanity check before running the script to avoid this happening by accident.