Sass client support + promise based processors

- [x] Ready for merge

Also re-implement client side stylus (ref #902).

Also optimised the render process, so if the source hasn't changed, then the compile process is ignored.

Todo:

- [x] Server side support
- [x] Reinstate stylus server side
- [x] Ensure processors run in `childProcess.fork` to protect parent
- [x] Fixes #1495
- [x] Test local install (without zeromq)
- [x] Investigate `@import` support
- [x] Show errors in panel like js
- [x] Rename `port` in `address`
- [x] UI for language dropdown menu
- [x] Debounce the call to the processor
- [x] Update Stylus to latest builds for client side and server
- [x] Add myth
- [x] Update sass labels in dropdown menu

Nice to have
- [ ] Possible UI for adding libraries/frameworks

Deferred (in priority order - issues to be created):

1. Syntax highlighting in CodeMirror (not sure if already there)
- Fade "Convert to CSS" out unless a non-CSS language is selected. Ref #1511
- Show processed output in an non-editable panel