`dash-at-point-with-docset` for choosing a docset from completion read

I've added an alternate defun for when you want to quickly select a docset to search with the word-at-point (or manually with the prefix arguement). This is handy for searching within frameworks (like jQuery or underscore, for example) that aren't discoverable by file extension. I also created a list variable called `dash-at-point-docsets` for adding to this global completion list.