Add option to `doctest` individual functions

It'd be nice if one could do something the following at the REPL:

julia> begin
       """
          f(x)

       add `1` to `x`
       ```jldoctest
       julia> f(1)
       2
       ```
       """
       f(x) = x + 1
       end
f

julia> doctest(f)
┌ Info: Testing the documentation for f: total 1 test
└ Test passed!
true

This way, one can use the doctest functionality without having to do all the other things documenter seems to require like having the function in a package, making doc directories, etc.