attr-sorted doesn't sort unknown tags

[chrisguttandin]

Is your feature request related to a problem? Please describe.
The attr-sorted rule sorts a couple of known tags but doesn't care about those which are not on the list.

Describe the solution you'd like
I think it would be great if all tags would be sorted in a reproducible manner. Tags which are not on the list could for example be sorted alphabetically.

Currently both versions of the custom-element below pass the rule.

<custom-element custom-a="a" custom-b="b" />
<custom-element custom-b="b" custom-a="a" />

I think it would be better if there was only one correct way to write this. When sorting other tags alphabetically that would be:

<custom-element custom-a="a" custom-b="b" />

Describe alternatives you've considered
It would be possible as well to create a completely new rule which sorts all tags alphabetically.

Additional context
I'm happy to provide a PR if you think this is a useful addition to HTMLHint.

[chrisguttandin]

I went ahead and created a PR to make sure this doesn't get stale. :-) I chose to update the existing attr-sorted rule. Please let me know if I should better create a new rule.