(back burner) Simplify commands

[GeorgLink]

Goal: reduce amount of typing needed.

Ideas:

• dates expressed as reference to now: e.g. +2d for two days in the future

[andyl]

I added a DSL for offer maturation dates (see #17).

At some point we'll need GraphQL for flexibility (see #29)

offer create_counter and offer take simplify things quite a bit.

Post suggestions if you have other ideas!