Pressing `e` while looking at a file in the _Status_ view will launch an external editor with the current file opened. The editor chosen is determined by the default logic introduced in #114.
An improvement to this in the future could be launching at the specific line at which the _Diff_ view is focused, but that seems to require a change in `FileDiff` which is a change bigger than this PR.
Fixes#166
Adds support for editing commit messages in an external editor. It read the GIT_EDITOR, VISUAL, EDITOR environment variables in turn (in the same order git does natively) and tries to launch the specified editor. If no editor is found, it falls back to "vi" (same as git). If vi is not available, it will fail with a message.