Support parsing via TryFrom<MetaItem>

[TedDriggs]

It would be useful to write something like...

pub struct Lorem {
    #[attire(meta_item)]
    ipsum: Dolor
}

impl TryFrom<syn::MetaItem> for Dolor {
    type Error = String;

    fn try_from(v: syn::MetaItem) -> Result<Self, Self::Error> {
        // do stuff.
    }
}

In parallel, it'd be useful if prom-attire could generate such implementations. This would make it easier to create reusable parts that adhered to the normal attribute format.

[TedDriggs]

A related enhancement would be #[attire(meta_item="path::to::function")]. That would enable overrides for cases where the type was remote. This might best be done with a trait defined by the prom-attire crate and a blanket impl of that trait for TryFrom<syn::MetaItem>.