Add an environment variable `ICDIFF_DEFAULTS` to allow setting default flags

[mr-salty]

I'm not 100% happy with this because there's no way to override boolean options using optparse. i.e. if ICDIFF_DEFAULTS=-N there is no way to override that on the command line.

Switching to argparse would allow that via BooleanOptionalAction but that requires python 3.9.

With optparse, I could also add no-XXX flags for all the existing boolean flags - LMK.

I use the exact string I put in the README FYI. I set that up in my VCS but I'd like to have it in effect all the time.