Include the defaults for keyword arguments

For example, DocStringExtensions produces the signature

iterate(ivp; solver, opts...)

for the following function definition

function iterate{S<:AbstractSolver}(ivp::IVP;
                                    solver::Type{S} = RKIntegratorAdaptive{:rk45},
                                    opts...)

In some cases this might be enough, but it would be helpful to be able to generate an output including the default value for the keyword arguments like this

iterate(ivp; solver=RKIntegratorAdaptive{:rk45}, opts...)

Possibly with a switch to turn this behavior on or off.

Start by tracing how DocStringExtensions turns Julia function definitions into displayed signatures, focusing on keyword arguments and their defaults. Confirm the existing output for the example, then define how an opt-in or opt-out switch should behave and verify that the generated signature can include solver=RKIntegratorAdaptive{:rk45}.