Expose significantCompressionPercent for user config

[reidab]

We'd like to be able to configure the percentage compression that's considered significant to avoid some images being double-compressed on subsequent PRs.

[benschwarz]

Thanks for sharing this @reidab. This seems like a pretty reasonable change to make. I was interested in learning what percentage you've set this to on your repositories?

Because image-actions is perhaps best when it's turned on and forgotten about, I wonder if it'd be useful to change the default percentage, rather than making it configurable. Keen to hear your thoughts on this.

[chabad360]

@benschwarz I could see this being useful, especially if #30 merged since a lot of metrics want maximum compression possible, even if it's only 1-2% it would still be helpfull.

Also, this is going to need to be modified now the #29 was merged...

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