Unhilight current line when in "edit mode"

[sampumon]

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.

[rchl]

I had no clue what this option is supposed to do before looking at the issue. Which means the naming is not good. But honestly, I think we could instead allow auto value for trailing_spaces_include_current_line to toggle that behavior. That seems cleaner and more understandable to me.