Make 'output' optional

[zanerock]

Overview

Let's make the output arg optional. If not present, spit results to stdout. This is useful for something like eval $(bash-rollup index.sh) or source <(bash-rollup index.sh)

Deliverables

• Add --output|-o option and set the output file based on this.
• For now, keep support of output as second argument, but emit a warning: WARNING: support specifying file output as the second argument will be removed in a future release. Use the --output option.