Add HTML format

#### What problem are you trying to solve?

We're currently outputting either a table or a json. A cool way to visualize the complexity/churn ratio would be a graph of files. HTML + d3js or something like that should do the trick.

#### Describe the feature

Adding a `--format=html` flag to the cli that generates a `public` directory containing a ready to open `index.html` with the graph.
