Provide more options for registering triggers

[wez]

The following options are desirable:

• set stdin, stdout, stderr or other descriptors, > or >>
• control over passing the list of changed filenames via argv (for big trees this list can exceed the command line limits of the system)
• control over passing structured data about changed files via stdin (again, can be large, and it may not need to be JSON for some traditional unixy command compositions)
• cwd, environment

To a certain extent, these can be controlled via a shell script that does the same, except for the command line length and JSON processing (unless you have installed some auxiliary json tools)

[wez]

This is failing in the TravisCI build but not in my Linux VM or on my Mac.
I suspect this failure is somehow Travis specific and I'm trying to get more info about the cause.
I'm waiting for Travis to pick up my latest build, will resume probing in the morning.