Add init --unzip

[fujiwara]

refs #471

Implements init --unzip.

If --unzip is specified, lambroll downloads a zip archive (same as --download) and extracts it into the directory (--src).

      --function-name=            Function name for init
      --download                  Download function.zip
      --unzip                     Unzip function.zip and delete it
      --src="."                   Source directory for unzipping function.zip
      --jsonnet                   render function.json as jsonnet
      --qualifier=QUALIFIER       function version or alias
      --function-url              create function url definition file
      --force-overwrite           Overwrite existing files without prompting