Support custom meanings for stop/start in .repl

[SevereOverfl0w]

Related to #2. It would be nice to allow users to re-define what it means to stop/start. e.g. which lifecycle methods to use. This would make it possible for user-space to add-in #1 without losing the managed state & exception handling that's bundled in (although it should be investigated whether that's still useful!)

[SevereOverfl0w]

.repl refers to clip/start and clip/stop in 3 places total. The only real coupling between .repl and clip.core is the assumption that ::clip/system will be in any exceptions, and that clip/start and clip/stop would be used as the entrypoint.

There's a deeper question of whether .repl is the right place to tackle this, or if the SPI should be at a higher level, rather than pre/post being prescribed.

[SevereOverfl0w]

#2 may fix this by allowing specifying :start, etc.