git-gl - Allow multiple selections

[Austin Gill]

• When copying hashes, copy all selected hashes, separated by newlines, to the clipboard
• When cherry picking, cherry pick all selected commits, in order, allowing for resolution of merge conflicts
• When rebasing with multiple selected commits, each selected commit should be marked as "edit" in the rebase todo file
• When pressing Enter with multiple selected commits, run git show on all of them (at once)

See #13 as well.