Allow WMAP plugin to delete site by index

WMAP plugin currently lacks the ability to delete sites, meaning
that console users have to remove sites via IRB/Pry. Given the
indexed output of wmap_sites -l, the index is used in delete_site.
If required, deletion by resolving the url,host format can be
added.
indentation compliance
Merges 'sempervictus/wmap_delete_site'

Adds a -d switch to wmap_sites

[Closes #1561]