Add meta functions around steps for the use of extra logging and profiling

[jeroenvandijk]

One of the main things that is missing in complex flows is the timing of steps. It would help for debugging purposes if we could add meta functions around steps. We could for example add extra logging around each step. Or log timing information; how long did it take to complete a certain step or a group of steps.

This idea is similar (but different implementation) to Prismatic's profiling function: https://github.com/Prismatic/plumbing/blob/master/src/plumbing/graph.clj#L280