Support omitting format string

[andrewhickman]

For example println!() or writeln!(stream) would just write a line break.

This mirrors the functionality of the std macros, making these more of a drop in replacement. Note the write_with_newline clippy lint prefers this short form, which can be annoying since it also triggers when using writeln from this crate.

[LukasKalbertodt]

That's a good point. Not quite sure how I did not do that in the first place.

This is fixed in the linked PR and released in 0.2.6.