Custom search engines

first working version of search engine support for [#1009](https://github.com/philc/vimium/issues/1009). I'm submitting this for comments and to check it's going in the right direction. 

# TODO:

* add separate search for just by searchEngines (I need to ask what key shortcut you would like? I'm thinking `s`
* Testing (I know I should have used TDD but was familiarising myself with codebase) as there is currently none.
* change user interface to be in line with [this outline](https://github.com/philc/vimium/issues/1009#issuecomment-42396773). The current UI is a simple box that is parsed.
* documentation of the feature