react-a11y-anchors should warn when there is no `href` attribute

[astorije]

At the moment, this plugin warns on <a href="#">Foobar</a> but it doesn't warn on <a>Foobar</a>. Thoughts on making it warn for this case as well?

For comparison, eslint-plugin-jsx-a11y's anchor-is-valid warns on those:

<a />
<a href={undefined} />
<a href={null} />

[sjgllghr]

If no one else is working on it, I'd like to look into this.

Just to be sure, should this test fail now since we're requiring href, or should it depend on role?
https://github.com/Microsoft/tslint-microsoft-contrib/blob/eac676244291b5dbb8720395d38e2a7929f7e427/src/tests/ReactA11yAnchorsRuleTests.ts#L105-L115