maybefloat & maybeint: parse string to Nullable

Introduces following methods that parse a string as the indicated type and return a `Nullable` with the result instead of throwing exception:
- `maybeint{T<:Integer}(::Type{T<:Integer},s::AbstractString)`
- `maybefloat32(s::AbstractString)` and `maybefloat64(s::AbstractString)`

Ref: discussions at #9316, #3631, #5704