"after" script for build.watch

[chrisfarms]

in #593 an after script config was added that lets you start up a longer running process like a watcher.

this is useful ... but for the "watcher" usecase, I don't actually want it to run after a "normal" ssc build only when I do an ssc build --watch

it would be good if we could also specify the after script in the build.watch section of the ini

[build.watch]
after = "my-customer-watcher"

currently I am working around this by setting an environment variable to tell my watcher script to skip: SKIP_AFTER_SCRIPT=true ssc build ... which is ok, but weird