Improve the Performance of nim-markdown

[soasme]

This is an umbrella issue for tracking the efforts on improving the performance of nim-markdown.

Below are potential bottlenecks:

• Use normal sequence, instead of doublylinkedlist for token sequence.
• HtmlBlockParser.parse is very slow. It has nested loop runs which can be optimized. Fall through to next html pattern early if tag startRe matched failed. #52
• Consume less memory/gc by assigning slices instead of string objects to tokens. Cache skipParsing #54, Performance optimization: eliminate substr calls in various places. #55, Performance improvement: remove firstLine & restLines calls. #56
• Pre-chop the string by lines, instead of ad-hoc splitLines.
• Use kind: XXX, instead of object inheritance.
• Ignore parsing content when an html comment is matched.
• remove bottleneck since() calls. Optimize string slicing. #53
• remove bottleneck firstLine() & restLines() calls. Performance improvement: remove firstLine & restLines calls. #56

[pietroppeter]

minor point related to #49: in the specific case of parsing html comments (<!-- ... -->), there is no need to parse the content inside (which I think it is currently happening).