Show the overlay in the bottom of the window too

[Wilfred]

This is useful when point overlaps the overlay at the top of the window, making it difficult to read the label.

Fixes #129.

Screenshot with many windows in one frame:

I've tried to match the code style of ace-window, but let me know if you'd like any changes.

[abo-abo]

Thanks, the change looks good, but some users will likely prefer to keep the current visual style. Please add a boolean defcustom that enables the new functionality, off by default.

[Wilfred]

Added a defcustom :)

[abo-abo]

Thanks.