Open file at location

This is my attempt to do opening file at location. I use : as separator for filename and line to jump, so to usage is calling  `light CONTRIBUTING.md:32` in terminal

I had to change trigger for `::open-on-args` to `:show`. Previously it could make a tab for a file during `:post-init` but focus on some other tab, so `:goto-line` happened in wrong tab. And I as a user expect that active tab would be with a file I pointed in command line.

I also want it to work in navigation bar. Similar behavior exist in Intelij IDEA and Sublime, you can add `:XX` after filename and editor jumps you to line XX. But I couldn't find easy way to implement it there. Need a way to ignore everything after colon symbol in navigation bar input field or something.