Add --custom argument

So that I can keep using the cli entry point rather than having to switch over to writing a dev server app, just to set this one option.

Example use case: https://github.com/jcorbin/flashphrases/commit/d14b30e74f2a17de364f7ca0f5c42b245072e80f

Also I added jshint setting to your project so that those of us with automatic editor integration of same don't get bogus warnings about your stylistic preferences.