Allow 20% less precision option to drop some final 1, 2, 8, 9

Created by: pborunda

Least significant digit that is equal to length precision can be safely rounded up or down if the digit is 1, 2, 8, 9. This is equivalent to 20% less precision so --set-precision 4 would behave like --set-precision 3.8

So --set-precision 3 3.593 is converted to 3.59, but its safe to round to 3.6 3.993 is converted to 3.99, but its safe to round to 4

This is not these same as --set-precision 2 3.593 and 3.553 both converted to 3.6 3.993 is converted to 3.953 both converted to 4

This actually helps shave off a few more bytes/KB for large svgs, when the difference between when going from --precision 4 to --precision 3 is visually not acceptable.

Right now i do this manually as an additional step in a bash script outside of scour, but I feel others might find it helpful to integrate to their projects.

I can submit a PR to demonstrate