Add a basic language switcher

[ozan]

@quackingduck can you review the implementation here please?

Fixes #14

Adds functionality to enable switching between languages.

To add language-specific content, wrap in a tag with data-language="clojure" for example. These tags are identified and the language switcher populated at runtime.

To test, play with the dynamic programming example

[ozan]

TODO: fix this

[quackingduck]

This looks great. From a user's perspective it would be nice if the code samples were labelled with the language they were written in. For example a little light gray py in the top right corner of each python sample. Because, at least the way I understand it, it's possible to be reading a chapter where not all the code samples are in the language the user picked from the dropdown.

[ozan]

it's possible to be reading a chapter where not all the code samples are in the language the user picked from the dropdown.

Yeah that's true. Something to add down the line.