New feature to make the mime types mappings configurable

Hi,

Now, it possible to customize the global mime types mappings and to
overload it for each virtual server. It can be done using following
directives in the global part or the server part of the configuration:

* default_type
* default_charset
* mime_types_file
* add_types
* add_charsets

Here is an example:

...
 default_type = text/html

 <server localhost>
   port = 8000
   listen = 0.0.0.0
   docroot = /var/www
   # nothing is overloaded in the vhost
 </server>

 <server localhost>
   port = 8001
   listen = 0.0.0.0
   docroot = /var/www

   # overload global configuration: