Tdewolff-minify

This is an incremental update over v2.1.0.

Changelog:
* CSS: properly handle bad declarations
* CSS: parse custom properties
* HTML: option to keep conditional comments
* Remove Content-Length in middleware
* HTML: attribute `spellcheck` is not a boolean
* JS: improved parsing of edge cases

CLI:
* Limit maximum concurrency for opening files to 50 (default)

This v2.1 release added a much improved SVG minifier. Equinox has been added to distribute binaries and make installation easier, see [Equinox](https://dl.equinox.io/tdewolff/minify/stable) (which says version 2.1.5). The CLI tool has matured and various bugfixes and feature requests are in this release.

Change log:
- HTML: do not keep IE conditional comments
- HTML: do not remove CDATA tags, as CDATA is invalid HTML anyways
- HTML: option to keep document tags (`html`, `head` and `body`)
- CSS/SVG: decimal precision can be set in the options
- SVG: more advanced minification
- export `MinifierFunc`
- new `Match` function that returns the minifier function that gets matched to a mediatype
- parsers return an error when they stop before the EOF
- `ResponseWriter` and `Middleware` functionality added

Bugfixes:
- HTML: whitespace around `img` and similar inline tags preserved
- HTML: whitespace within `pre` tag properly preserved
- HTML: the `math` and `svg` tags themselves are now parsed by their respective parsers
- CSS/SVG: valid rounding and truncating of decimals
- CSS: do not lowercase all identifiers, as they may be `<custom-ident>`
- CSS: `rgba` and `hsla` now work properly
- CSS: keep certain required number dimensions
- SVG: path data for arc; sweep and large-arg flags are now treated as booleans
- JS: keep newline or semicolon before `!function(){}`
- JS: keep newline before lines that start with a string in certain instances, the original JSMin does not handle this
- JS: keep newline between `}` and string or ident

CLI:
- Add concatenation support
- Add more support for file and directory watching
- Only overwrite files when explicitly set output equal to input, previously an empty output would imply overwriting, now it means writing to `stdout` as intended

This v2 releases introduces passing options to minifiers. This is a feature that resulted in an API change, additionally I did some API clean up as well. This works with parse-v2.0.0.

Furthermore, performance of several components has been improved. Tests have been added and improved.

Change log:
- options have been added
- `Reader` and `Writer` functions are added
- whitespace trimming doesn't convert newline to space if it can't remove it
- bang-comments are preserved
- improved CDATA processing
- HTML URL minification

Bugfixes:
- HTML: mediatype attribute in foreign/style/script wasn't copied
- HTML/XML: don't panic on early ending end tags
- SVG: allow single quotes attributes
- SVG: keep M command character

CLI changes:
- complete refactor allowing for a variety of inputs (streams, files, directories)
- more parameters such as watching directory changes, recursive minification and options for the minifiers
- processes files in parallel

This uses a more efficient underlying buffer which removes any need to copy byte slices. All minifiers have higher performance, bugfixes and test refactoring.

This denotes a stable release point which is guaranteed to work in the future. Bugfixes and development is done on this version, while promising a stable API. Meanwhile work is put into v2.0.0 which will support options passing.