Simplify

* Set up the watcher only once
* Rearrange some deck chairs
Watch all subdirectories
Put in a 50ms event coalescing period
Log the command being run
Check for errors when killing the process
Failure to launch is fatal
Wait for the process to complete before respawning
Fix compilation error introduced in 40d70aa44f9cdb73abd0016ac48c2505809559a2
Don't watch directories starting with _ or .
Merge branch 'master' of https://github.com/josharian/spot
build to /tmp/spotted-<exe>