The file field should not have whitespace collapsed, because file names may
actually contain multiple spaces in a row. See Github issue #33. Because
collapsing whitespace is done in `parsebib--expand-strings`, it was applied
automatically if @String abbreviations were expanded. This commit changes that.

Instead, only replace sequences of newlines, tabs and form feeds with a
single space.

This is in response to Github issue #33.

This reverts commit 3e2f46f5.

Also make the set of fields that should not be post-processed customisable.

Fix #28

Co-authored-by: Rahguzar <[email protected]>

The rewrite of the TeX markup clean-up functionality makes these two functions superfluous.

* Speed up markup removal

Authored-by: Rahguzar &lt;[email protected]&gt; / <[email protected]>