Add Transformer impls to handle handle Option<T> properties

[AlephAlpha]

Fix #768

For a component Foo with property:

#[derive(Properties)]
pub struct Props {
    pub string: Option<String>
    pub bars: Option<Vec<Bar>>
}

We will be able to write <Foo string="string" bars=vec![] />, which is equivalent to <Foo string=Some("string".to_owned()) bars=Some(vec![]) />. <Foo bars=None /> is still <Foo bars=None />.

[jstarry]

Looks great, thanks for all adding all the tests!