Fixes for the --watch flag for faas-cli up

[alexellis]

Fixes for the --watch flag for faas-cli up due to incomplete / defective feature PR.

Description

• Fixes issue where all functions are rebuilt even if only one changed
• Fixes issue where unrelated files cause rebuilds
• Fixes issue where vim can't trigger and update due to using create
• Adds newly created folders into the watch when within a function's path
• Triggers rebuilds via stack.yml

Motivation and Context

Fixes: #968

How Has This Been Tested?

Tested with:

• creating a new folder within a function's handler
• editing a function's code multiple times with vim
• editing the stack.yml file
• faas-cli new --append during a watch
• two functions, only the changed one is rebuilt
• Out of scope for this PR - "copy extra directories" that can be specified for each function

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[alexellis]

cc @nitishkumar71 @nitishkumar71 @welteki

[alexellis]

I've added a note on why --watch won't work with localrun with the current design.

We need some kind of Go routine, cancellation/context approach so that the docker run which is blocking, doesn't block the watcher code - and the watcher code has a chance to kill the docker run for a prior version before starting a new build/run.