Use the upstream color semantics for EasyMotion.

[Kharacternyk]

Though the changes made nicely fit into a small PR, they consist of three parts:

• The upstream suggests red highlighting for 1-letter targets, which works IMHO much better than yellow because of the contrast. This PR changes the highlighting to red.
• I expanded the coverage of EasyMotion highlighting to include 2-letter targets. The highlighting falls back to inconsistent with gruvbox default colors otherwise.
• The highlighting group for faded text currently points to the comment group, which contains italic styling. The switch of font is very distracting, so I made this group point to GruvboxGray, which is again similar to what upstream does by default.

If some of these points don't make sense, I will revert that particular change.

[rbong]

I haven't used easymotion for a while but the changes sound reasonable to me.