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

https://github.com/greshake/i3status-rust/blob/a2704aedebf8818628977cde99a5a1886e63e09f/src/formatting/parse.rs#L114-L116