Allow custom header code

[nox]

In every generated grammar_mod.erl, abnfc automatically includes grammar_mod.hrl. The grammar files should provide a way to include custom header Erlang code instead of doing that.

[nygge]

That mandatory include file is an artifact from an earlier version where it was used for including the "semantic" functions for each rule. It is on my TODO list to remove it and instead add to abnfc an option to specify any .hrl files to include.

[nox]

Why not an equivalent feature to the Header statement in yecc?

[nygge]

The header section in yecc is only usable for comments. It is included BEFORE the module declaration. What I had in mind was something like the includefile option to yecc:file/2. But it would be nice to have a header section that can contain any erlang code.