Add new Powerline-inspired theme powerlineish2.vim

Hello,
Powerlineish is a nice theme but I think it can be improved. So I tried to make it a little more readable and consistent.

Changed a few colors and made the rightmost section always match the mode indicator's color, while the middle is blue when writing (insert and replace modes) and gray otherwise.

Created as a new theme because I figured you wouldn't want big changes in existing ones.