Implement ParseOption::indented_multiline_values

I implemented indented multiline values. This should be able to parse most `ini` files that the Python `configparser` crate is able to parse. There are a lot of other things that the Python `configparser` crate is able to do, but there is no way we are quickly implementing all of it. I also decided against returning an Err in case of a multi-line key. I suspect that this will cause quite a few errors in programs that simply try to parse `configparser`-like ini files and where some of the keys were just ignored. I'm not sure how to proceed there. I would definitely release a new minor or major version, because it will break some programs. Fixes #138.