Tdewolff-minify

Bugfixes:
- SVG: don't convert polyline/rect/polygon/line to path, which can break CSS, fixes 260
- SVG: relative SVG Bézier commands not properly minified
- CSS: don't remove whitespace in nested unknown at-rules, fixes 262
- CSS: fix panic for `background` when it contains functions other than `calc` for `background-position`, fixes 263
- CSS: fix panic for `background-position` with three numbers
- CSS: fix panic for url() with only whitespace or only one quote

* Remove import comments
* SVG: do not convert line/rect to path if coordinates are relative percentages
* CSS: fix bug with inline CSS encountering `}`

* CSS: fix position information in error context
* CSS: fix `background-position` panic with offsets that are zero
* HTML: trim and collapse whitespace in certain attribute values
* SVG: shorten cubic and quadratic Beziér path data to their shorter format or to line segments

- When minifying floating points, remove trailing zeros and not other numbers
- Make `M` concurrent-safe
- HTML: keep double quotes around XML-flavour RDFa attributes
- CSS: revert the use of the `RRGGBBAA` format as proposed in Color Module Level 4 as it is not yet an official W3C recommendation
- CSS: `background-position` (also within `background`) now works with 3 and 4 values as well
- SVG: skipping `metadata` or `rect` tags properly even if they are closed by a void tag (like `<rect/>`)
- SVG pathdata: when cursor doesn't move, don't emit any commands

I'm suspecting that `go mod` gets confused from a previously reverted `v2.3.7` release, so just in case this is a new release that will hopefully fix:


$ go get -u github.com/tdewolff/minify/v2

Add Go modules support by using the `github.com/tdewolff/minify/v2` import path.

WARNING: this release may give errors for `go mod` probably due to (local) caching...