should not rely on absolute column numbers

[andiechu]

Getting data from spreadsheet should not rely on absolute column numbers.

Ricky has a function getHeaderMapping to enable lookup of header row -> header column number:

https://github.com/coderbunker/gas/blob/master/src/timesheet/data.js

turn that into a reusable library and have it in both

Originally posted by @rngadam in #65