Repl enhancements

[lzrd]

It would be useful if the repl could be used in scripting and/or provided scripting.

• Input from stdin (not just /dev/tty) should be accepted
• Comments and blank lines should be ignored
• All commands should respect the -j flag for JSON or other parser friendly output format.
• Ensure that humility commands' success/failure status can be parsed.
• Doing more than the above quickly gets to the point of embedding an interpreter, evaluate that effort and decide what to do.

This should work but doesn't because reedline forces input to come from /dev/tty and not stdin.

egrep -v '(^#|^$)' <<! | humility ...
# Get task status:
tasks

# Comments and blank lines should be ignored without filtering input.
!

[steveklabnik]

Re the reedline bug: nushell/reedline#509

[jclulow]

It would be pretty neat to be able to pass a rhai program to humility that was able to run commands, inspect their output, make decisions, cause the humility process to exit with a non-zero status code, etc. I suspect we could use that in manufacturing for things that we, today, perform with bash scripts that chain multiple humility invocations, etc. It could also be useful in a future series of automated dump classifiers.