New locators for content libraries

This PR implements two new types of locators necessary to facilitate content libraries. I'll be posting a separate PR to edx-platform for the content libraries prototype soon, but for now I'd like to start the review process for this locator code.

The new locators are described in detail under the "New Locators" heading at https://openedx.atlassian.net/wiki/display/SOL/Content+Libraries

I noticed this pattern would match URLs with a trailing plus, e.g. `course-v1:BradenX+PHYS200+2014+` which seems weird - with this change to add a lookahead, a lone trailing `+` is no longer matched. It's not important for the libraries, just something I saw while re-working to make `run` optional.