Add a UI and/or URL parameter for config file

[ebrelsford]

As @aparlato mentioned in #40, it might be helpful to let users modify the config for the app on the fly by changing it through a UI or by passing a URL to a config file.

If we do the URL way, we might actually be able to have project-specific versions of the tool without building and deploying a project-specific version--just point the app at a different config.