Added support for project:issue-number queries

Added support for project specific issues (Chromium is the still default when the project name is omitted). b:v8:989, b:skia:822 and so on.
This is not optimized at all (it is called several times per query - creating an object every time - instead of being cached and re-used), but so was the previous code (though it was much simpler).
I'd actually prefer mainProject_ here and projects_ below, but I was just going to make the change myself later to not delay accepting this. However, given you may need to rebase anyway, so might as well make this change.