Config modifications to suit working with a TiddlyWeb server

A couple of new configuration options.

The first, dubbed "tiddlyweb mode" allows pushing to custom bag names (i.e. not _public or _private as prescribed by TiddlySpace).

The second allows for a server prefix to be included in all URL requests, similar to `server_prefix` in `tiddlywebconfig.py`.

For example a GET using a prefix of "web" would look like:

    http://host/web/bags/bagname/tiddlers/tiddler