Fetch list of subjects directly from devdocs.io

[astoff]

Doing so avoids having to maintain a list of current subjects.

I actually have a few more suggestions for this package, and maybe at some point I would like to have it on MELPA as well. Would you like to receive more patches from me (and possibly bug reports from people coming from MELPA) or would you rather let my version diverge from yours? (I'm asking this based on your response to https://github.com/skeeto/devdocs-lookup/issues/11.)