Operator pending

This is a rough implementation of `operator-pending` mode, with some explanation and discussion in #58 and #129.

Based on discussion in #129, this doesn't seem like it's the correct long-term solution to the problem.

All tests pass in this PR except for two new tests that came after I put in the original work for this mode that I haven't had a chance to look at --

```
Operators
the y keybinding
when selected lines in visual linewise mode
it saves the lines to the default register
```