Problem: need way to customize client API

There are some methods which are too painful to generate, as we
don't yet have clean abstractions for them.

Solution: allow user to add custom code to generated API, using
<custom> tag in client model. Syntax:

    <custom filename = "somefile.xml" language = "C" />

where somefile.xml contains <header> and <source> sections included
into API header and API source engine respectively.