Add Markdown support

[elrido]

@wware suggests to add Markdown support to ZeroBin, i.e. based on the library used in this showdown demo.

As @Hexalyse had suggested in #21 to have a user option for the current code highlighting, we could implement it with a dropdown having options for "plain text", "source code" and "markdown".

[marsjaninzmarsa]

...and IRC colors, because why not (I can provide it) and there is no other pasta service doing this. ^^

[marsjaninzmarsa]

sebsauvage#91, also.