Support specific block ranges in seth-logs

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