feature: add watch mode to faas-cli up workflow

[LucasRoesler]

Expected Behaviour

We want to add a flag to the faas-cli up command so that it can run continuously and re-build and deploy when files are changed.

For example faas-cli up --watch will build and deploy the functions in the stack.yaml and then continue watching for file change notifications and rebuild and deploy the stack when an event is seen.

Current Behaviour

faas-cli up is a one-off workflow right now.

Why do you need this?

Make the dev-loop shorter.

Context

This was discussed in the community call as a feature that we would like to have and it would be a test ground for also adding the watch behavior to the faas-cli local-run

[LucasRoesler]

For the file notifications, skaffold is using the fsnotify package and it looks like a solid package.

[alexellis]

Here's the equivalent command with Google Skaffold which is well known for giving an "auto-build" workflow:

GoogleContainerTools/skaffold#8396

Example function / config for Skaffold with faas-cli: https://github.com/alexellis/openfaas-skaffold-faas-cli

After we have a working prototype for faas-cli up, local-run would probably be a good place to go next.

See also as related: #953

[alexellis]

An example with a library that doesn't need CGO and it's used by Skaffold:

https://github.com/alexellis/fstail/