Allow variables to use Go templating syntax

[brikis98]

This PR updates boilerplate with experimental support for allowing variables to use Go templating syntax. This allows, for example, for one variable in Boilerplate to reference another:

variables:
  - name: Foo
    default: foo

  - name: Bar
    default "{{ .Foo }}-bar"  

[brikis98]

Merging now so I can test this out with the reference architecture. Feedback welcome!