FYI: another extra for managing headers

In a project I'm working on, I wanted to be able to capture twitter-style hash tags, but still have markdown format. However, if the #tag was the first thing on a line, it would match the header regex, which wasn't what I wanted. So, now when the extra 'tag-friendly-headers' is passed, whitespace is now required between the opening # and the rest of the header.  Otherwise, the #tag is left as-is.

Example 1:
#tag
standard code: <h1>tag</h1>
w/ my extra: #tag

Example 2:
# header
standard code: <h1>header</h1>
w/ my extra: <h1>header</h1>

Interestingly, this change doesn't break any of the tests... nor is it explicitly dealt with in Gruber's syntax sheets (that I know of).

Feel free to accept or reject the pull as you'd like.

Commit text
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Requires white-space between "#" and the header in order to qualify as a "<h?>"

This lets us include hash-tags (#tagname) as the first item on a line and have it *not* converted to a <h?>