Adding offer feature: partial fills

[thederpinator]

It would be a great next step for liquidity on the offer markets to add the feature of partial fills of offers. I read about the aggegration of offers in the docs, but can't find the possibility to partially fill orders.

Market makers

• Intoduce option to allow partial fills when creating an offer
• Able to set minimum quantity of partial fill

Market taker

• Specify quantity for partial fill of offer