disable pMcode

As detailed at http://ellislab.com/forums/viewthread/102409/, I needed a way to disable pMcode processing in posts on a client site. This experimental extension does that by converting all square brackets in text sent to the Typography class into the corresponding character entities ([ and ]), so the pMcode conversion doesn’t happen.

This doesn’t seem like an ideal solution, and this might have some unintended consequences elsewhere in EE (like intentional pMcode in the control panel not being converted?), but try it out and let me know what you think and if you have any suggestions for improvement.