Support partial serverless.template

[peter-james-chapman]

It is difficult to mange the serverless.template as a single file, it becomes cumberosme to navigate and find the appropriate sections. Including long strings e.g. database queries or emails directly in the JSON requires escaping line endings and quotes, this makes them difficult to edit in place.

Allow multiple serverless.template files (or a precursor file type) to be created and then combine these into a single template on publish. Add a substitute function that allows strings to be pulled from a co-located .txt file, escaping quotes and line endings.