Allow targeting a second HTML file

[garrettmflynn]

commoners should allow users to easily specify multiple applications that rely on the same base configuration.

Different approaches to this could be taken, including:

1. One config for both applications + html files in the same root + specify the input html file in the API / CLI.
2. Two configs and html files in different roots, where each config inherits from the same base config—or one is the base.

Example Application

Manage a pair of clients that render a shared public display (desktop) that can be interacted with using a personal private controller (mobile).

[garrettmflynn]

This feature has been completed in the sense that you simply import the base configuration and run each application separately.