Allow --line-range to count from the back of the file to act similar to "tail"

[PlasmaHH]

For python users it would probably best if -r acts somewhat like the python syntax...

So for negative numbers it could count from backwards, so "tail -n 5" would be "bat -r -5:" or so and then one could display the first and last portion of a file like "bat -r :5 -r -5:"