How to force recompile?

- `babel-plugin-preval` version: 1.4.1
- `node` version: 8.1.3
- `npm` (or `yarn`) version: npm 5.2.0

Relevant code or config

```javascript
const result = preval`
const fs = require('fs');
const path = require('path');
const content = fs.readFileSync(path.join(__dirname, 'file.md'));
module.exports = content.toString();
`;

console.log(result);
```

What you did: I ran the code above, to export contents of `file.md`.

What happened: when I modify `file.md` the result of `preval` didn't change when I tried to compile it again. Maybe I'm missing something?

Reproduction repository: [demo](https://github.com/silvenon/demo-babel-plugin-preval-recompile/tree/master)

Problem description: `preval` probably doesn't see the reason to recompile because its tagged template literal didn't change.

Suggested solution: I don't know. 😅

Start with the linked demo reproduction and inspect how babel-plugin-preval processes the `preval` tagged template and reads `file.md`. Compare a rebuild after changing `file.md` with the unchanged template, then determine what behavior should trigger recompilation. Done means the generated result reflects updated file contents after recompiling.

Written by the indexing model from the issue text.