Add '--search-paths' option

[zanerock]

Overview

Instead of making the 'search dirs' optional trailing args, support a 'search-paths' option of comma separated path names.

Deliverables

• Implement --search-paths as source for optional search directories.
• Update 'usage' documentation.
• Use bash-rollup --help output to grab the updated documentation and update the README.md to match. Note that the README.md makes use of markdown formatting so the raw text is slightly different.