add settings.php

-create new file to set options, this is included in letter.php
-parameterize latex engine
don't add the 'token-' prefix to placeholders

Up until now, the server side added the prefix. In order to prepare
support for optional tokens, this is now done by the client.
remove white spaces from placeholders to prevent security issues
only allow tokens which have the form 'token-*' or '%opt-token-*'
template: make subject an optional token