Support for .jsig files

JSig is a type documentation format for JavaScript (
  https://github.com/jden/jsig ). It can be considered
  somethign similar to typescript definition files or
  to jsdoc.

The documentation format would be a lot nicer to read
  if it had syntax highlighting. Rather then adding
  a full jsig lexer, i've set the lexer to OCaml as its
  a close approximation.

You can find examples of jsig in the wild here:

 - https://github.com/Raynos/dom-delegator/blob/master/docs.mli#L1
 - https://github.com/jden/node-random-revisitor#randomrevisitor--functionerror-serviceurlstring--void
 - https://github.com/Matt-Esch/virtual-dom/blob/master/docs.mli#L1
 - https://github.com/grncdr/node-any-db-transaction#apix

Let me know if I can do anything to make this PR easier
  to review or merge!