Add a generic open external link file plugin

I have some file (ex: pdf) that I want to link in my file but sublime cannot open them. Adding a file plugin with [[file:]] could send a request to the os to open a file. For example on osx the call would become "open " and let the os figure it out.

The issue proposes handling [[file:]] links by asking the operating system to open the referenced file, with macOS's open command as an example. No file, test, or entry point is named, so first locate link handling and plugin tests; done means a file link opens the requested file through the host OS.

Written by the indexing model from the issue text.