Created a plugin for our code tags

[stranac]

The plugin has been reactivated, and all the bugs previously mentioned seem to have been resolved.

• [php] and [code] tags are now happily pretending to be [python]
  
• [error] and [output] tags now have newlines (the MinifyHTML plugin was causing problems)
  
• [icode] tags only work on a single line
  
• symbols (©, ™, ®) re-enabled
  
• preview displayed correctly (the problem was just css not being included in the preview page)
  

If anyone notices any further problems,let me know.