Thanks! There is an option in the settings menu to "Avoid hills", and in the not-too-distant future that will become a slider that allows you to prefer hills or avoid them.

It's not open source yet, but I might open it up in the future. There's a donate link hidden away in the About page. Any donation would be much appreciated, and would help with the server costs (it needs a huge amount of RAM to store the whole planet's data).