Add functions to update displayed files
Add functions to update displayed files

- Track diagnostics per file path - only for certain file types
- As window displays new file highlight diagnostics
- When highlighting diagnostics highlight in every window which has the
  files open
- Pull out a method for clearing existing highlights
- Add a utility method to run windo without changing the focused window

TODO
- For now treat 'vim' as the only tracked file type (easiest to test on
  the files I'm editing for the plugin). Make this configurable

natebosch/vim-lsc,natebosch/vim-lsc