Make enum metada

[josueBarretogit]

At the moment when calling epub_builder.metada it receives key which is AsRef<str>, it returns Err when you don´t pass a valid key, wouldn it be better to have an enum Metada so that you can´t pass an invalid key? btw I really love this crate you guys made it so simple to make epubs, I thought this was gonna be like using a pdf crate but this one turned out to be really simple to use.

[duyk471]

Sounds great, I've already added the PageDirection Enum for PageDirection:

impl ToString for PageDirection {
    fn to_string(&self) -> String {
        match &self {
            PageDirection::Rtl => "rtl".into(),
            PageDirection::Ltr => "ltr".into(),
        }
    }
}

This is only applied for values that have fixed options, such as Page Direction, EPUB_VERSION (I guess there are only two of them, maybe some other properties such as "primary-writing-mode" which is needed when generating Japanese and Chinese ebooks, but they're pretty niche so I guess they won't be added soon)

[crowdagger]

Yes it would be good, might look at it when I have the energy :)