Allow omission of end option for range reads

Problem: Sometimes it is useful to read a file from a certain position
to it's end. The current implementation was already perfectly capable
of this, but decided to throw an error when the user tried to omit
the end option. The only way to do this, was to pass {end: Infinity}.

Solution: Automatically assume {end: Infinity} when omitted, and remove
the previous exception thrown. Also updated the docs.

closes #801.