Use `getproperty` with field matching

Requesting an update to #3 to use `getproperty` instead of `getfield` to work with calculated fields.

Example:

```julia
struct Foo
    a::Int
end
function Base.getproperty(x::Foo, f::Symbol)
    f == :b && return getfield(x,:a) + 1
    getfield(x,f)
end

@match Foo(2) begin
    Foo(a = 1) => :a
    Foo(b = 3) => :b
end

```

This should return `:b`.
