Support logging macros to a file

[brunchboy]

Create a function similar to the one for persisting cue variables, which appends to a chosen file Clojure code which can re-create a macro whenever the user defines one. Rather than auto-loading, this file is for people to scan and choose to incorporate pieces they like into their show definitions whenever they have created a particularly neat macro.

[brunchboy]

This has been written in 30905aa, but needs to be documented and generalized a bit so the Push can use it too, and perhaps made easier to activate—it probably even deserves its own command-line argument.