Add SyncTeX support

This pull request should add SyncTeX support for the Skim opener, with keymap `ctrl-alt-s` (syncing on current line).

Probably these things need to be looked at before merging:
- [ ] Instructions for the Skim side of configuration (perhaps contact Skim authors to get a preset in Skim settings)
- [ ] Does keeping track of the PDF file name work? Is it persistent across sessions? Is it per project/file or global as implemented?
- [ ] Tests?