Fix #5

Before:

>>> ghc backtrace 0x00007ffff1911d64
usage: [-h] [-d DEPTH] [-n FRAMES] [--sp SP]
: error: unrecognized arguments: 0x00007ffff1911d64
$

After:

>>> ghc backtrace 0x00007ffff1911d64
usage: [-h] [-d DEPTH] [-n FRAMES] [--sp SP]
: error: unrecognized arguments: 0x00007ffff1911d64
>>>