Feature: Add template save stack

[martindekov]

PR https://github.com/openfaas/faas-cli/pull/668 introduces a way to configure your templates in a configuration and run command in order the templates to be pulled, but the change does not provide command which will save your configuration.

Expected Behaviour

Add template save stack commands to provide easy configuration of the templates throug the cli, instead of adding raw fields to the yaml file.

Current Behaviour

Once pulling logic is merged(the tagged PR), there is no way to configure templates to your file through the cli, you must manually fill the yaml file where your configuration lives.

Possible Solution

Add template save stack with mandatory flags --name and --url which will append template sources to your configuration.

Steps to Reproduce (for bugs)

N/A ATM.

this is a feature, extension of Issue #617

Context

This is needed for tracking purposes.