Provide a formatted_yaml template handler

[alex-harvey-z3q]

Subject of the issue

When generating templates from Jinja2, it is difficult and time-consuming to use Jinja2 line trimming to produce a neatly-formatted output template. This often leads to messy code seen in commands like sceptre generate.

It is proposed to add a template handler formatted_yaml that subclasses the File template handler and reformats white spacing and line breaks in a standard way.

[zaro0508]

this is done with the sceptre-formatted-yaml-handler. thanks @alexharv074