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 don't understand why you added `?=.` which seems to be a noop: "there may be a plus sign here or anything else. If plus sign, eat it, if it's anything else, don't read it in" seems equivalent to "there may be a plus sign here. If so, eat it."