Inline markdown

[megamaths]

Added inline markdown like ability to add italics bold and monospaced (via */_, **/__, `) through the new md attribute instead of the text attribute.

[codedstructure]

In testing, inline code format doesn't work fully, e.g. md="test `a*` test" should show a* in monospace, but doesn't. Try testing against the github comment preview window!

The markdown parsing / splitting should be extracted to a separate markdown.rs file.

[codedstructure]

I think a basic example in examples/markdown.xml would be useful, even something as simple as the following, though multiple lines etc would be better.

<svg>
  <rect wh="50 10" md="**Hello** _Markdown_ `world`!"/>
</svg>