Support multiple files

[zanerock]

Overview

Why limit ourselves to a single index file? Note, this is a breaking change

Deliverables

• Process all positional arguments as files to be included (concatenated) one after the other.
• Update usage.sh.
• Use bash-rollup.sh --help to print usage and update README.md with changes. Note that README.md is markdown formatted, so the raw text may vary to achieve the appropriate formatting. Also, make any links live.

Notes

The 'unique' inclusion rule should apply to all files. E.g., if file A and B both include foo.sh, then the contents will be inlined at A, but not B (unless flagged with bash-rollup-always-inline).