Support $ interpolation when parsing docstrings for JLFunction

[0x0f0f0f]

julia> :("foo")
"foo"

julia> bar = 42
42

julia> :("foo $bar")
:("foo $(bar)")

julia> dump(ans)
Expr
  head: Symbol string
  args: Array{Any}((2,))
    1: String "foo "
    2: Symbol bar

julia> ex = quote
       "foo $bar" 
       f(x) = x+1
       end;

julia> JLFunction(ex)
ERROR: MethodError: Cannot `convert` an object of type Expr to an object of type String

How to do that? We can simply change doc::Maybe{String} to doc::Maybe{Union{String, Expr}}.