Add h4 to h6.
Add ref handlings for markdown.
Use ropes instead of repeated string concatenation

Reduces lambcmd.native time on a 200kB document with lots of RAW tokens: 10s -> 1.5s
Repeatedly concatenating lots of RAW tokens is an O(N^2) operation and generates
a lot of extra work for the garbage collector too.
Use BatText instead which doesn't concatenate immediately but uses ropes.

(Could also use a string list and concatenate at the very end, but BatText is clearer)
Add Img supports for markdown reader.
Merge pull request #13 from Drup/better_markdown

Improve  markdown handling
Merge pull request #12 from edwintorok/use_ropes

Use ropes instead of repeated string concatenation
Formatting and documentation tweaks
Bug fix: support all 6 levels in Writeconv
Disable warning 48
Pervasive support for six section nesting levels