Allow Inline HTML

[filiph]

Not sure if this is in scope, but markdown normally allows HTML tags. You can write something like this:

This is **markdown'd**. But you can also <span color="red">do stuff manually</span>.

The span should be copied verbatim to the HTML output. In this implementation, though, all HTML tags are escaped (< becomes &lt).

[dpeek]

This is supported in the block parser, but not inline:
https://github.com/dpeek/dart-markdown/blob/master/lib/src/block_parser.dart#L325

I'll take a look at it.

[0xcaff]

@dpeek Since dart is primarily for the web, we should add a safeHtml option to this library.