Connect selected text to STDIN of exec'd program

[Dave MacFarlane]

When running a program with enter or middle click, it currently only runs it with the stdout pipe set up to capture the output and insert it. It should also be passing the text (or file) along stdin so that people can, for instance, pipe the file to gofmt.

This probably also requires making some kind of alternate "Dot" to use for middle click/enter, because otherwise clicking changes the selected text. (But that's going to be required for Cut and Copy anyways..)