Better multi-language support in the build system

Right now we need 3 identical copies of each documentation page, which need to be kept in sync. This is really error prone, and I'm sure super annoying for @chipotle. With the upcoming creation of an official java driver (rethinkdb/rethinkdb#3930) slated for 2.1, the work is about to increase 33%.

Ideally, what it would be really nice to see are some custom tags that we can pre-process into the final files that jekyll uses (if there's some way to make this a jekyll plugin, even better)

Hypothetically:

In the {{#py "python"}}{{#rb "ruby"}}{{#js "javascript"}} driver, we do things like ...

{{#py}}
The python driver uses tornado underneath, so we create generators...
{{/py}}
{{#rb}}
Eventmachine is a wonderful way to do magical delightful ruby things...
{{/rb}}

Having per-language tags as above would be a nice start. Obviously there are some more convenient things we could do, like having a special tag like {{#lang}} that outputs the current language's name, since that will probably be common.

@mglukhovsky @AtnNn thoughts?

Start by examining the Jekyll build path that produces the final documentation files and determine where a preprocessing or plugin hook could be added. Define how the per-language tags and proposed {{#lang}} tag should render, then verify that one source page can replace the three synchronized copies.