[patch] Headers, unresolved footnotes, and fractions

[Matthew Sachs]

Hi folks, thanks for the useful code!

I've added some functionality to PHP Markdown Extra and PHP  
SmartyPants.  Diffs attached.

For PHP Markdown Extra:

*A new config option, MARKDOWN_HEADER_BASE, which sets the largest  
header level that Markdown can generate and defaults to 1; useful if  
you use <h1> for site-wide elements and want Markdown-generated  
headers to start at <h2>.  It will use <h6> instead of trying to  
generate <h7>, <h8>, &c.

*A new config option, MARKDOWN_STRIP_UNRESOLVED_FNS, which will cause  
unresolved footnote references to be stripped instead of left in the  
output text uninterpreted and defaults to false; useful when Markdown  
is being run over post excerpts or if you use features like <!-- more  
--> to only show part of the post on the front page.

For Smartypants:

*1/4, 1/2, and 3/4 will be replaced with fraction entities.
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