initial functionality, -i supresses error when invalid rev used
I believe invalid revs are now handled properly with the -i/--invalid-revs flag.
Don't suggest invalid next_rev. Don't check for rev skip if in --invalid-revs mode.  Reference CI_Sheet in any cell/row based warning or error, instead of ECO.
expanded invalid rev warning text
tweaks to errors & warnings
removing placeholder for pnr update, openpyxl module won't work in current form