Add screencasts to the main page

It really attracts people.

I've prepared fish completions for `salt` https://github.com/saltstack/salt/pull/15310 and prepared screencast (gif, using Byzanz). It's a good way to demonstrate features of fish (picture itself says much less).

I think it could look like this: http://dimsemenov.com/plugins/royal-slider with 3-4 short screencast showing autocompletion, multiline editing, fish web configuration.