new plugin: reduce id names to the minimum

[deepsweet]

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[kizu]

I can see it be not that safe: if someone would try to use SVG Stacks one day, the obfuscated IDs could break things up, 'cause author would no longer know what IDs are there.

However, if there are some common ID patterns used by SVG-producing software, it would be not that unsafe to reduce them.

However, I can see how an external Stylesheet could use those IDs. However, there can be a lot of other problems with external CSS, so I don't think it'd be a problem there :)

[deepsweet]

yes, I thought about this issue, and probably it will be the first plugin disabled by default.
even is there are a lot of autogenerated ID like "SVG001" or "G57" there is no any guarantee that they are not used outside.

maybe svgo should ask questions like "do you use SVG id attrs somewhere? [y/n]" :)