Allow multiple facets to be selected in search

[andrew]

Allow user to filter by Ruby or Rust, same for licenses, keywords and platforms

[BenJam]

Been taking a look at this as part of #1366 but stumped. Looks like:

https://github.com/librariesio/libraries.io/blob/search/app/views/search/_term.html.erb#L7 should add term['key'] to an array rather than an individual key and https://github.com/librariesio/libraries.io/blob/search/app/views/search/_term.html.erb#L5 needs to remove term['key'].

I've added a clear method should this be implemented in 839b571

Interestingly if you try to craft a URL for this this the new key is added to the filters but ignored.