Add get_mut support

[Technohacker]

Mutability support is opt-in by using the #[typemap_mut] attribute

Fixes #2

[Technohacker]

I was initially planning on just adding get_mut to Get, but I realized that would be a breaking API change since it could potentially introduce mutability where it isn't wanted 😅

Added benefit with it being opt-in is users can choose to keep their structs immutable. If the need arises, it should be possible to add type-selective mutability by specifying the attribute on the field instead of the struct, but that isn't implemented in this PR :)

[stephanemagnenat]

Thanks a lot! Lints fail due to formatting. Please run cargo fmt and then I can merge.

Also, I guess you'll need a new release. I assume we could release 0.1.1, but maybe it is better to release 0.2, do you have an opinion on that point?

[Technohacker]

Will do!

To my understanding this would be new functionality without backwards-incompatible changes, so 0.2.0 sounds good to me :)