Add new Powerline-inspired theme powerlineish2.vim

Hello,
I created a new theme because I figured you wouldn't want big changes in existing ones.

I found the original Powerlineish hard to read in some places so I tweaked it a bit. 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. Also a small addition for bufferline.

Would you consider merging this?