Add perl formatting support using Perltidy

[liuyehcf]

Solves #210

[dbarnett]

Sorry, I think since the tests weren't passing this fell through the cracks. I think if you can rebase your changes on top of the latest upstream and address my comment there to fix the tests, this looks ready to merge otherwise.

[dbarnett]

You need to fake the command output like this:

vim-codefmt/vroom/clangformat.vroom

Line 46 in c6730fb

$ { "Cursor": 0 }

Otherwise the tests can't run without perltidy installed (and may get brittle, failing depending on the version of perltidy installed).