A more liberal markdownValid

It appears Markdown uses the following regex to find the beginning of a tag:

    <[a-z/!$]  # Markdown.pl:1276 from version 1.0.1.

This means, for example, `<=` would not be a considered a tag.

Currently, it *is* considered a tag. Curiously this can sometimes lead to the behavior witnessed in #29, where things kind-of look okay until you use a `'` somewhere after your `<`. The `'` then has the syntax stack `htmlTag -> htmlString` and things go all wacky. (Well, things already *were* all wacky, since everything after the `<=` is considered internal to a tag.)

This patch makes markdownValid more liberal, looking a bit more like Markdown's regex. It may fix some cases of #29.