add function to install missing documents

[Kaligule]

This implements the suggestions from #34 (comment)

This is what I am missing to finaly use devdocs daily: A way to declare all the documentation I want.

Usage:

(use-package devdocs                                                                                        
  :bind (("C-h d" . devdocs-lookup))                                                                        
) 
(devdocs-ensure-docs-installed '(html css javascript))  

This is important for me because I often use emacs offline. Then I need the documentation, I can't download it on demand. This will install the documentation without doing it manually on every new setup of emacs.

[Kaligule]

I would also consider adding a simple minimal config to the Readme, something like the above usage block. I would like to hear your opinion on that first, though.