Minor markdown fixes
- Fix images with alt text
- Add support for code blocks in html and markdown
- Fix indentation and line breaks in code blocks converted from html and markdown.
As the markdown to bbcode conversion goes via html, this also fixes the same issues when converting from html.
There's some fundamental issues, both with the way markdown and html is converted to bbcode, so the support for code blocks is not really usable yet. I'll address that in later patch sets.
This patch may break some html or markdown input that was working before. Should probably be tested more before merged.
Edited by Harald Eilertsen