Add whitespace trimming configuration for Stencil templates

[jgongo]

Stencil provides a way to configure whitespace trimming when creating the environment used to load the templates. In particular it allows you to configure leading and trailing whitespace trimming with several options.

It would be great to have some way to configure Sourcery whitespace trimming when using Stencil templates, so this configuration can be forwarded to the Stencil environment. (AFAIK there is no way to do it currently)