Review request for Typora app

Hi, we are developers of Typora.

Typora allow users to run custom scripts for export or upload images, etc. So we want to have read access to /usr/bin and /usr/local/bin, and added system-files plug for this purpose. Please review it.

The yaml file is at https://github.com/typora/typora-snap/blob/master/snap/snapcraft.yaml