Allow slicing from index to end of string

[chtenb]

In the javascript slice function the stop parameter is optional, which makes the slice function slice until the end of the string.
This doesn't seem to be possible in the purescript version, since both parameters are mandatory.
There doesn't seem to be a slice version taking only one index argument.

[natefaubion]

There is drop/take to slice from the beginning or to the end.