Split up grammar spec to multiple files

[sebastianriese]

For complex grammars it may be desirable to separate the lexer and parser to multiple files.
The problem: When using stokens they are fully interdependent. So a syntax like:

# header section
%lexer("path/to/lexer.pyLR")
%parser("path/to/parser.pyLR")
%footer
# footer section

or

%include("path/to/include.pyLR")

might be the best solution. The %include version is also interesting for parsers and lexers sharing structure (sample usecase: adding support for actions in other languages to the self hosting pyLRp parser: most of it will be similar, or even the same, but the lang-blobs will have a different lexic/grammar).