Set output format at config.yaml Level

[njsnx]

Coming from using Sceptre v1 a lot to 2.0 I have noticed the move towards favouring text output as the default output_format rather than JSON as it was before.

It took me a little while to figure out that a lot of my commands looked broken as I was not setting an --ouput flag on my commands to JSON (or YAML) and since getting used to it, It has occurred to me that at a project level and even at a Stack level, it would be really useful to be able to set this once and not have to worry about setting a flag each time.

I would like to be able to set this using something like output or output_format at my config.yaml files and then utilise this as my preferred option over setting --output.

Cascading config should still work and maybe if --ouput is set, it overrides any value in the config.

[njsnx]

PR Raised to get this implemented. #673

[ngfgrant]

relates to #152