MD051 should support arbitrary id attributes, not just <a> tags

[MarkLodato]

MD051 currently only checks <a> tags for id="..." attributes, but this requirement seems arbitrary. Any HTML element may have an id attribute to define an anchor.

It seems like this can be easily fixed by removing the check for a tags on line 58.

In my case, I have a table and use id attributes to link to specfic rows of the table:

# Example

<table>
<tr id="foo"><td>Foo<td>Foo is good
<tr id="bar"><td>Bar<td>Bar is also good
</table>

I like [foo](#foo) and [bar](#bar).

$ npx markdownlint test.md
test.md:8:8 MD051/link-fragments Link fragments should be valid [Context: "[foo](#foo) and [bar](#bar)"]
test.md:8:8 MD051/link-fragments Link fragments should be valid [Context: "[foo](#foo)"]

Would you be willing to accept a PR that fixes this, or is there some other reason why only a tags are accepted?

[DavidAnson]

Yes, I would accept a PR for this! Please read the CONTRIBUTING guide first. Thank you!