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

[waymondo]

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.

[stanaka]

Thank you for the pull request!