allow user to specify files and directories to pack in release

[tylerflint]

Currently, the go engine assumes the project will compile to a single binary executable. As web frameworks emerge and assets, templates, and other build objects start to be more common, this will need to be expanded to allow the user to specify specific files and directories to include in the packed release.

[glinton]

Until there is a better way, users can manually copy files into the packed release (build) in the deploy.config extra_steps bit of the boxfile

run.config:
 engine: golang
 engine.config:
   fetch: go get ./...
deploy.config:
 extra_steps:
   - cp -r $CODE_DIR/assets $APP_DIR/assets

Note: not to be confused with run.config's extra_steps