Add new experimental picker preview highlight engine

[liuchengxu]

With Vim native syntax highlight (space-vim-theme):

With syntect syntax highlight (Visual Studio Dark+):

In my opinion, this new highlight engine is not superior to the native vim syntax highlight with regard to the highlight quality and speed.

• Vim's syntax theme is more sophisticated than Sublime Text's.
• We only render the area in the window so either way is expected to be pretty fast.

This PR is a step towards #532