Allow specifying a DocumentRoot

I am developing a [little script](https://github.com/Cottser/devsite) which relies on virtualhost.sh but searching for a matching folder just won't cut it in many cases. The changes here are pretty minimal, the main thing is setting `$FOLDER` to `$2` (trimming the potential trailing forward slash) and wrapping the whole folder search code in `if [ -z "$FOLDER" ]`. All the other changes are whitespace and documentation.

See also #44.