Support import paths via `inline` instead of inline mdx

[hamlim]

From a recent discussion on twitter: https://twitter.com/kentcdodds/status/1026817174717968386?s=20

It would be cool to be able to "import" mdx files without needing to use webpack:

API:

import React from 'react';
import {inline} from 'inline-mdx.macro';

const MDX = inline('./my-markdown-asset.md[x]');

render(
  <MDX />
);

I assume we would need to wrap our current "quasis" logic here which is handling tagged template literals in a conditional to verify that the user has in fact used the tagged template string vs use inline as a function and then fs.readFileSync the file and parse it as MDX.