Allow to customize puppeteer config

[Worie]

Hi, I've started working on glyphhanger within docker so it is not required for developer to set up python, pip, font-tools etc in each development environment that he/she may work in.

Working with puppeteer in docker can be tricky and the fact that currently it is not possible to provide a config for puppeteer which glyphhanger internally uses has forced me to fork the repo, add the flags that I needed and use the fork within docker.

This is bad, because now I need git in the image to install the forked glyphhanger, which increases the size of the image and increases initial installation time.

I think that option to provide a path to a JSON file which would be read, and later used within await puppeteer.launch() is the best thing that could be done, but I am not sure how would you like to approach this issue (and if) - thus I didn't create a PR

Currently, I am using this fork https://github.com/Worie/glyphhanger/tree/no-sandbox (branch no-sandbox) to make this docker image work: https://cloud.docker.com/u/wopolow/repository/docker/wopolow/glyphhanger

And btw, thanks for the amazing glyphhanger lib, it's just so clever! :)