Make chalicelib optional

Hi, thanks for writing chalice, it's sweet.

I would like to request making chalicelib optional in favor of explicit include/exclude paths to be deployed. This could certainly default to using chalicelib for deployment, but I think some people would prefer a more configurable/flexible solution.

With chalicelib/app.py as the only way to deploy your own code, you either have to add chalicelib to sys.path (so you can get relative imports, e.g. https://github.com/aws/chalice/issues/299), or litter your imports with "import chalicelib.yourlibrary", etc. Also, if you have optional files which should remain just in the dev deployment with .gitignore, there's no way to keep those in the chalicelib tree without deploying them. Sometimes keeping them out of chalicelib can be messy too.

I'm familiar with the discussion in https://github.com/aws/chalice/pull/146 and would like to vote for the explicit include/exclude option.