Add hz command to generate custom horizon client builds

[deontologician]

This would build with a couple different config options:

• Set the horizon server explicitly
  • Sometimes (probably usually in production servers) the static domain and hz domain are different. This would bake the horizon server's location into the client itself.
• Include or exclude Observable methods
  • Right now we export a barebones observable. It would be convenient to let the user patch methods onto the observable
• minimize or not

For maximum configurability, the user should use webpack themselves, but this might make some simple use cases easier. The idea would be to wrap the pre-built source code that we plop into dist/ in the npm package with a simple wrapper that injects the config options.

Quick sketch of what I mean:

var fileContents = `(function() { var hzConfig = { /* some properties here the module will look for */ };` +
readFileSync(require('@horizon/client/dist/minimized').filename) +
`})()`

Note: this is explicitly with the goal of not having webpack as a dependency of the cli

[deontologician]

Superceded by #638