added scripts for generating cross-costreport comparisons

[mwarin]

This time without a buncha commits from other branches!

Copied and modified from https://github.com/hathitrust/print_holdings, a couple of scripts that look at old cost reports and generates 3 files:

1. A history of total-cost per member per report
2. A history of total-cost diff (month to month) per member per report
3. A history of total-cost diff percentage (month to month) per member per report

These all go into the finished product, the Google Sheet, as separate tabs.

[aelkiss]

Looks OK to me for now. If we find that we need to add a feature or otherwise make a non-trivial change to append_sheets.pl or cost_changes.sh, I think it's worth translating those to Ruby and getting them under test.