Interpolation in W_cmd

[BeastyBlacksmith]

Currently you can't interpolate into W_cmd's, quoting from the PR:

W`Sin[$(1+1)]` # W"Sin"(2)

The main challenge with this is that $ is allowed to be used in a variable name (see >here). I'm not sure how best to do this, but perhaps open a new issue to discuss it.

Originally posted by @simonbyrne in #19 (comment)

[BeastyBlacksmith]

Instead one can

W"Set"(W"n", 1+1) |> weval
W`Sin[n]`

[simonbyrne]

An alternative would be to provide keywords to weval:

weval(w`Log[1+x]`, x=x)

by wrapping the expression in a Module, Block or With (I'm not sure which one would be best). This would be pretty nice combined with JuliaLang/julia#34331.