Add get_option attribute

[hochej]

Implements #78

Hi @Hoverbear, first of all thanks for your work!
I stumbled across the same issue as mentioned in #78 and tried to implement the suggested solution.
This PR adds a new attribute get_option to getset, so that the getter function returns Option<&T> instead of &Option<T>.

Note the following syntaxes are supported (and tested) for Options:

• Option
• std::option::Option
• ::std::option::Option
• core::option::Option
• ::core::option::Option

However own type aliases like type Bar<T> = Option<T> are not supported.

The following code:

use getset::{OptionGetters};
#[derive(OptionGetters)]
#[getset(get_option)]
pub struct Foo {
    field: Option<usize>,
}

derives

impl Foo {
    #[inline(always)]
    fn field(&self) -> Option<&usize> {
        self.field.as_ref()
    }
}

Unfortunately, I have little experience with procedural macros so far and hope that I have implemented the suggestion in an acceptable way.

[hochej]

I noticed that the same behavior, as described in #74, occurs in this case.
This means it is not possible to use field level attributes to override the global attribute for get, get_copy or get_option.

[Hoverbear]

Yeah, I'm kind of convinced global options was a mistake. :(