vCard property group support

[rolisv]

Hi @Peltoche

It looks like property groups are not supported.

From the spec https://www.rfc-editor.org/rfc/rfc6350#section-3.3 on page 6:

...
   contentline = [group "."] name *(";" param) ":" value CRLF
...
   group = 1*(ALPHA / DIGIT / "-")
...

And then page 7:

The group construct is used to group related properties together.
The group name is a syntactic convention used to indicate that all
property names prefaced with the same group name SHOULD be grouped
together when displayed by an application. It has no other
significance. Implementations that do not understand or support
grouping MAY simply strip off any text before a "." to the left of
the type name and present the types and values as normal.

So having vCard property line:

gr1.FN:Bob

The expectation would be something like:

let p = vcard.get_property("FN").unwrap();
assert_eq!(p.name, "FN");
assert_eq!(p.group, Some("gr1"));

IMHO, the key point here is that the property name should be "FN" and not "gr1.FN".

Is this functionality being considered? I could submit a PR.

Cheers