include-files.lua: Support for environment variables

[mfhepp]

It would be super-handy if the filter supported the use of environment variables and the other shortcuts supported by e.g. the Pandoc defaults files syntax so that one could reference files from other directories with ease.

```{.include}
${MY_TEMPLATES}/disclaimer.md
```

or

```{.include}
$MY_TEMPLATES/disclaimer.md
```