Custom extension mapping

[treycucco]

Overview

Adds the ability to provide a custom extension to code type mapping.

Background

Currently the code blocks are tagged with the extension of the file they come from. For instance if there is a best practices block in a .tsx (TypeScript JSX) file, the code block will be

```tsx
// code block
```

However some documentation systems might not recognize tsx as a valid type and instead want a simple `ts'

Description

This PR adds a way to define extension-to-code-type mappings on the command line via the -m, --extension-mappings CLI argument, such as:

-m tsx:ts jsx:js

This will cause .tsx file code blocks to have a ts type, and jsx file code blocks to have a js type.