A complete refactor

Shower is great but building from HTML is too trivial. Jekyller hadn't been updated since March, leaving, as @kizu said,

> a lot of things to refactor

With these commits, we have a completely-reviewed-and-refactored Jekyller. Here is a done list:

- Update all the submodules, namely `shower/core`, `shower/ribbon` and `shower/bright`
- Update *_config.yml* to catch up Jekyll's changement (also fix issue #32)
- Add .slide automatically for all sections in layout, leave others to be added manually in markdown (fix #34 and #8)
- Implement figcaptions in the way metioned by @pepelsbey (fix #16)
- No syntax for `<mark>`, but to highlight codes by pygments (related to #14)
- Use collections to manage all examples and slides (fix #7), and create a index page of them (fix #5).

I kept all the docs unchanged and look forward to your suggestions.