how to append syntax rule for certain file type?
# Problems summary


## Expected

syntax for python variable type annotation
[PEP 526](https://www.python.org/dev/peps/pep-0526/) 


![image](https://user-images.githubusercontent.com/5415510/37344833-64a4f9f4-2706-11e8-99d4-9b9b97e19e5a.png)

## Environment Information
 * OS: linux
 * vim version: 8.0
 * neovim version: 0.2.2


## The reproduce ways from Vim starting (Required!)

 1. add syntax file `python.vim`
 2. render when ft=python

