Alternative output formats for print-schema

[killercup]

smag suggested graphviz output for print-schema on Gitter, and I think this is a nice idea. I'm not entirely sure if this is something we want in diesel_cli itself or if it'd be nicer to have a new crate that does that (it could use diesel_infer_schema as a dependency).

[sgrif]

I'm pretty sure there are other tools that output database schema in graphviz. This really seems out of scope for Diesel though.