Unhilight current line when in "edit mode"

Unhilight any trailing space on current line(s) when editing them. This is a compromise between the possible modes of include_current_line setting.

Might be a bit hacky with global dicts saving stuff, but noticed similar hackiness with other globals already present :)

Set `"trailing_spaces_unhilight_editing_line": true` to enable. Note that it does its thing only if `trailing_spaces_include_current_line` is true (which it is by default).

Fixes #42.