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Adding support for GeShi highlighter.

It must be dev-master because of your stability setting. using "dev" there would work with geshi >= 1.0.
That is why I used that one here.
People can use dev as stability and then require >= 1.0 then if they want to.

I wonder, though, how both Pygments and Geshi could have settings passed with the current setup.
We should probably allow customization. Either with additional options (array), or with events/hooks.
Or add a DI approach for this, where you can pass in your "post parser".
Options sounds easiest and could be: `new ...Extension($options)`.
Why - if it is optional. Wouldnt that throw fatal errors then if the packages for it are not included (as they dont have to be).