feat(attr-sort): sort unknown attributes alphabetically

[chrisguttandin]

Previously unknown attributes were just ignored. Now they get sorted alphabetically.

BREAKING CHANGE: HTMLHint will now throw an error if unkown attributes are not sorted.

fix #661

Short description of what this resolves: This PR will make sure all attributes get sorted when using the attr-sorted rule. It will ensure that attributes linted with this rule will always get sorted the same way.

Please let me know if there is anything that should be changed to make this PR mergable.

[stale[bot]]

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[coliff]

Heya @chrisguttandin - can you please resolve the conflicts so we can get this one merged? Thanks!

[chrisguttandin]

Hi @coliff, I've rebased the branch on top of the current master.

[chrisguttandin]

@coliff Is there anything I can do about the failing CI tasks?

[coliff]

Sorry for the delay - I think we need @thedaviddias for assistance on this.