re-frame changes incoming

[mike-thompson-day8]

This is advanced warning that we are planning to make two changes in re-frame, probably to be released in the next week to 10 days:

• dispatch can be given a map, not just a vector
• subscribe can be given a map, not just a vector

For further details, please see day8/re-frame#644

[mike-thompson-day8]

There have been some modifications to the plan. I have edited the issue above, to remove stale detail, and provide a link to the ongoing discussion.