Formatting: treat key as a shortcut for key:true

[MaxVerevkin]

For example .dur(hms) instead of .dur(hms:true).

[bim9262]

I like the idea, but I think we'd want to only do that, if the parameter is a boolean. Otherwise we'd get odd errors for args like arg_takes_number, arg_takes_string

[MaxVerevkin]

Funny thing is that the comments in parse.rs suggest this was the intention all along

i3status-rust/src/formatting/parse.rs

Lines 114 to 116 in a2704ae

	// `.str(width:2)`
	// `.eng(unit:bits,bin)`
	fn parse_formatter(i: &str) -> IResult<&str, Formatter, PError> {