Please support relative paths in --ignore-dir

[ittayd]

I normally want to ignore the folder target under the current one as it contains build artifacts. But ignoring any folder named 'target' anywhere is "dangerous" since I may miss something (I use ack a lot to grep on source files of thirdparty projects). It would be good to be able to specify --ignore-dir=./target to ignore just the top one.

[petdance]

See also #43 and #329