Fix emacs one-liner file vars having surrounding whitespace stripped

[Crozzers]

In #454, I introduced a bug with emacs one-liner file variables that caused surrounding whitespace to be removed from them.
This issue is demonstrated by the following markdown:

---
key: value
---

-*- markdown-extras: metadata -*-

The emacs one-liner would be matched and substituted by _emacs_vars_oneliner_sub, which would not replace the \n\n between the markdown metadata closing fence and the one-liner, resulting in the following text:

---
key: value
---<!-- -*- markdown-extras: metadata -*- -->

Which would later go on to cause an IndexError in _extract_metadata.
This PR fixes this issue by extracting the leading and tailing whitespace from the matched emacs one-liner and making sure they are added back to the one-liner after it has been wrapped in HTML comment tags

[nicholasserra]

Thanks!