Add window commands

Add window commands

- `w`: Open new window.
- `W`: Open new private window.
- `gw`: Move tab to new window.
- `gF`: Follow link in new window.

Fixes #402.

In #402 two more commands were discussed:

- `q`: Close window. Can easily make more harm than good. Also, why learn a
  Firefox-specific shortcut to close a window, when you can use an OS-wide
  shortcut?
- `Q`: Suggested only for symmetry with `x` and `X`. YAGNI.