Improved performance by adding a search api in addition to the current way of fetching files

There is a new interface in the main menu to search for the required plugin using the API suggested in https://github.com/dafrancis/Sublime-Text--cdnjs/issues/26,  before the GET request is made to the cdnjs server. The HTTP response is much smaller which makes the plugin perform quicker.

This could replace the older interface entirely, but i'l leave that up to the community to decide. Feedback is welcome.

Also added a menu item for the plugin under Preferences -> Package Settings -> Sublime-Text--cdnjs