Fix babel-plugin-htm behavior for non-element roots

[jviide]

This pull request fixes issue #104, making babel-plugin-htm act correctly for non-element roots. For example text-only roots (html`foo`) or mixed text/dynamic roots (html`foo${1}bar`) should now work. This is done by unifying how root nodes and child nodes are handled, essentially simplifying the plugin code.

This PR also add tests for the fixed behavior (that fail without the fix).

Fixes #104.

[luwes]

@jviide was if (node === null) return t.nullLiteral(); not needed anymore?

[jviide]

@luwes Correct. The node parameter should not be null in any valid situation, it should be either undefined, a string, a Babel node or an HTM element (of form { tag, props, children }). Unless I'm mistaken, of course 🙂

[luwes]

thanks, great to know. Congrats on the 5000 🌟 This pkg is the best thing since sliced bread!