Add search functionality

[atavakoli]

In order to use this beyond tutorial/walkthrough use-cases, we need the ability to search the contents of a gitbook. Having an optional searchbox in the navbar would make this a near complete tool for generating documentation sites.

[SamyPesse]

Since it generate a static website, we could create an index.json file containing an inversed index word -> [list of files] that can be used for showing a searchbox.

@AaronO What do you think?

[AaronO]

I think it's a good idea. I'm going to flesh this out in a branch and submit a PR.

I'll do all the backend and index generation. @SamyPesse I'll let you take care of the frontend.

[atavakoli]

Thanks guys :)