connectionString support in `FromJSON` for `PostgresConf`

Would the maintainers be open to a PR expanding the FromJSON implementation for PostgresConf so that it accepts a connectionString value as an alternative to passing database, host, port, user, and password individually?

I generated a new Yesod application using stack new foo yesodweb/postgres, and the first thing I wanted to do was get the generated code up and running on Heroku. Heroku provides the postgres connection string as a single environment variable in a libpq-compatible format, so it would be convenient to be able to pass that same value straight through to the configuration. Since the template-generated starter code from stack new uses the FromJSON implementation for PostgresConf as the basis for configuration with environment variables, it seems like expanding the FromJSON implementation would be a sensible way to add connectionString support.

I worked around this with newtype to get something working in my own app. I'd be happy to submit a PR with the FromJSON changes.

Start by locating the FromJSON implementation for PostgresConf and the template-generated configuration path described in the issue. Compare its existing field parsing with the requested connectionString input, then add coverage showing that a libpq-compatible value is accepted while existing individual fields continue to work.