Implement ParseOption::indented_multiline_values

[davidhalter]

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.

[zonyitoo]

ParseOption::indented_multiline_values would be renamed to enabled_indented_mutiline_value for naming consistency.