Initial support for practicing with code samples

[rydnr]

This patch adds preliminary support for speed-typing with code samples, should the user chooses so.

The changes can be summarized as follows:

• Introducing "extensibility points" in the speed-type code which use gb-specific functions.
• Added some defcustom to allow the user to choose if the text used to practice would be extracted from english texts, or from a list of github urls. There's now a defgroup "speed-type" and two defcustoms (for choosing among english or code, and to customize which urls to take code samples from).
• Some preliminary functions to properly display Java code snippets for practicing.

I've tried not to mess the original code up too much. The "extensibility points" are just switch-case blocks to choose alternatives according to the user setting. Not very elegant, but easy to understand. However, for supporting additional types of content it'd probably need a cleaner approach than the suggested here.