Support specific block ranges in seth-logs

[travs]

Right now seth logs --block $N $CONTRACT yields logs from $N to the latest block.
I expected it to just print the logs in $N.

Maybe it makes sense to add --from-block and --to-block as constraints for the range, and the current --block=$N could be shorthand for --from-block=$N --to-block=$N.

This may also affect other commands that use --block, like events.

@nanexcool