Mime types: Use x- prefix for non-standard internet media types

To comply with RFC2046, internet media types that were not registered with IANA must begin with the `x-` prefix.

I also took the liberty to suggest adding a shorter `text/x-lisp` variant to the `text/x-common-lisp` mime type.

CC: @shaunxcode, @dqminh, @ComFreek, @FordLawnmower, @Hardest, @bsletten, @peteguhl and @steveohara, please verify that this doesn't utterly break the apps using your modes. If it does, and you really can't fix them, please veto the change before it gets merged or ask for a revert afterwards.