add singleton option to prevent concurrent child processes

[hbt]

Current behavior:

• start watcher on a directory and execute a command when a file changes
• change a file
• command execute
• change file again
• another child process executes in parallel

This is problematic in cases where the command is a long build or anything that shouldn't be running in parallel.

Current solution:

• add a locking mechanism within the script to ignore the watcher

Desired solution:

• watcher should have a --singleton option which waits until a command and its child processes are done running before executing it again