Add a combinator to choose between the least greedy of two parsers

The syntax is `a / b' which will run both parsers and return the
results for the one which consumes the least characters.

Also update line numbers in README.md (is there a better way to keep
track of this? could link directly to anchor text of some
documentation)
Add decorators that convert functions between calling conventions

Since state functions in tparse must take the state as an array, it is
convenient to be able to write functions that take positional
arguments and just wrap them with the utility wrappers provided
Update document parser example

moc.py is now powerful enough to generate tparse's documentation