Support with tag in templates

[christophermaier]

From @vanstee on September 28, 2016 13:41

it's common to want to conditionally change scope, similar to how mustache templates work. Let's implement a with tag to do exactly that. Here's an example:

~with $command.bundle.author as=bundle_author~
  Author: ~$bundle_author~
~end~

which is essentially the same as:

~if cond=$command.bundle.author~
  Author: $command.bundle.author~
~end~

Copied from original issue: operable/cog#996