Add plugin options handling

[antongolub]

Hi there.

I'd like to add optional plugin configuration. There are a couple of solutions:

1. options as JSON (this PR)

--options={"plugins": [["pngquant", {"quality": [0.2, 0.5]}]]}

2. options as properties

--plugin.webp.quality=95
--plugin.pngquant.quality.0=0.2 --plugin.pngquant.quality.1=0.5

Which looks better?

[sindresorhus]

Definitely 2.

[antongolub]

Done. There's also a useful side effect: now each plugin is loaded once.

Вefore:

imagemin --plugin=pngquant --plugin=pngquant --plugin=pngquant
↓
{
 ...,
 plugins: [
   require(`imagemin-pngquant`)(),
   require(`imagemin-pngquant`)(),
   require(`imagemin-pngquant`)()
 ]
}

After:

imagemin --plugin.pngquant.quality={0.1,0.2} --plugin=pngquant
↓
{
 ...,
 plugins: [
   require(`imagemin-pngquant`)({quality: [0.1, 0.2]})
 ]
}

[antongolub]

@sindresorhus, any objections / suggestions?

[antongolub]

@sindresorhus, is this good enough to be merged eventually? )

[antongolub]

@1000ch, @kevva , could anybody review this PR?

[toufali]

Eagerly awaiting this merge 🙏

[sindresorhus]

Thanks, @antongolub 👍