Expose full guru interface API

The current `plan_*` API does not expose the full functionality that the C FFTW API has, namely the ability to specify a custom stride pattern and the number of transforms to perform.

In my use case I have dense arrays of a custom number type `VarNum{T}`, wrapping two `T`s, and need to perform transforms on the `x` and `y` components separately. For instance, consider:
```julia
struct VarNum{T<:Number} <: Number
x::T
y::T
end

# arrays
u = Matrix{VarNum{Float64}}(10, 10)
U = Matrix{VarNum{Complex{Float64}}}(10, 6)

# create and apply plan
p = plan_rftt(u, [2, 1])
A_mul_B!(U, p, U)
```

I have somehow hacked into `FFTW.jl` code as much as needed to achieve what I want. This required doubling the strides of `VarNum` arrays, doubling the `howmany` parameter, and defining appropriate methods for `VarNum` arrays. I feel, though, that such functionality might be useful to many other people and it would be good to have it in the package.