Create text-rendering module and remove custom markdown2 dependency

[Stefan4472]

In #38 I changed the rendering process to use a modified version of the markdown2 library. But actually, I now think there's a better solution that doesn't require changing the library (markdown should stay markdown):

• Take file
• Read into beautifulsoup
• Split out the custom <x-...> tags and render them separately
• Render the rest of the text as Markdown by default
• Merge the Markdown-rendered stuff with the custom-rendered stuff

If desired, we could even have <x-markdown> tag and <x-html> tags, for example, but that would require migrating all existing posts and I think default-Markdown rendering is a good idea.

This should also fix #40

Put this rendering code together into a separate Python module that we can import into both the backend and the CLI.

[Stefan4472]

Merged and deployed.

[Stefan4472]

Need to update the README and requirements.txt