Allow to use negative line indices to reference lines from the end (e.g. 0 or -1 for the last line)

[vlsi]

Samples:

1:-1 -- focus from the first to the last line. It would be equivalent to 1:5 in case the code has 5 lines
2:-2 -- focus from the second line to the last but one. It would be equivalent to 2:4 in case the code has 5 lines

WDYT?