Do not add duplicate extensions in MIME type table

Assume extensions in /etc/mime.types are in a certain order of importance.
Reduce MIME type table even further by not adding padding bytes

Compressed data isn't much smaller, but uncompressed data is ~3x smaller
this way.
Allow setting Expires header timeout in configuration file

Format is simple: just a number and its unit. More than one pair can be
specified.  Supported units are seconds (s), minutes (m), hours (h), days
(d), weeks (w), months (M), years (y). Unknown modifiers are ignored, and
configuration still defaults to one week.