Parse orderbook to propose values for order size and fees

As briefly mentioned in issue #14 and [Call 22](https://github.com/joinmarket-webui/jam/wiki/Community-Call-%2322), it would be great to have an automated mechanism that looks at the order book, and suggests sane values to the user.

* How big are the current offers?
* How high are the absolute/relative fees?

Might be interesting for both makers and takers. Suggestions on the maker side, fee transparency on the taker side.