Multiline Adminitions not converted correctly on export

[dom8509]

Problem: The regex pattern in the export script is only looking for adminitions terminated by a newline character. This does not work if the adminition extends over several lines.

Solution: Included newlines in the regex, excluded newlines from the pattern where they're not needed and terminated the search pattern by the string #+END_. Hope this works for all cases.

[sawhney17]

Looks great! Thanks a ton for the fixes!