Add more highlight code options

[richardscull]

Currently, there is a very limited number of supported code highlighting options, for example it currently lacks support for C#.

Since zipline uses hightlight.js, it shouldn't be a problem to add support for more languages.

I understand that users can edit code.json file by themselves to add more languages, but it seems very redundant to me, and any additions/deletions to this file should be handled in the admin dashboard at least.