adding JSON5 support

I got tired of seeing [this](https://github.com/aseemk/json5/blob/develop/package.json5) without syntax highlighting.

Not adding samples.json changes since rake task reordered the entire file (44859 additions, 44810 deletions, see [samples branch](https://github.com/rlidwka/linguist/compare/samples)), so I thought you don't want to have that in your git history.