POC: Cli command

[ChristianDavis]

This is a rough POC for generating maps from the CLI, related to #71. Ideally we'd be able to load a file into the JS, return HTML or an SVG, then pass that to a renderer and save an image. But right now the browser is the render engine.

I tried OWM -> React -> html2canvas directly, but the browser was required.
Tried running everything in JSDOM (which could potentially work), but html2canvas doesn't support SVG, relies directly on the browser for that.

This version loads the app in a headless chromium instance and then uses chrome's screenshot tool. Didn't need to screenshot cause the html2canvas save function works this way as well, but managing the file was more involved.

This is actually a workable solution but the full chromium instance is a little overkill. The only caveat to this PR is that it needs the dev server running. The need for the local server can be eliminated with a small build step and rendering the built files (or committing some built files), but I didn't go through the webpack config for that.

to test:

yarn install
yarn start

in a new tab, run the cli command

yarn cli -f maps/example.owm -o maps/example.png

Try it out and let me know what you think