[Radiant] Markdown extension using RDiscount

[john muhl]

I've patched the default Markdown extension so it uses RDiscount  
instead of BlueCloth. The point was to improve the accuracy of  
Radiant's Markdown conversion; for an added bonus it's also really  
fast. Although the difference in speed diminishes as the amount of  
Markdown to convert goes down, so if you just have little bits of  
Markdown here and there, skip it, if you're publishing lengthy  
articles or pages heavy with Markdown behind the scenes then you'll  
probably notice a significant difference. See the links at the end for  
a demonstration.

The only down sides are that it requires the `rdiscount` gem to be  
installed on the server and a little change to the order extensions  
are loaded. The github page has all the details. It should be noted  
that the change to extension load order is probably becoming the  
standard order eventually -- I think unless some reason not to pops up.

If none of that's a problem for you then head over to github for the  
download and a bit more information.

There is also the `markdown` branch in my radiant fork where I am  
trying to get together something that could possibly, eventually be  
applied to the core; so if you have the `rdiscount` gem installed you  
get its Markdown filtering and if not you get the standard BlueCloth  
rendition.

Any preference between the standalone extension and getting the  
`markdown` branch into the core?


[bluecloth]: http://johnmuhl.com/test/markdown-bluecloth
[rdiscount]: http://johnmuhl.com/test/markdown-rdiscount
[github]: http://github.com/johnmuhl/radiant-markdown-extension
[radiant]: http://github.com/johnmuhl/radiant