Specifying output file to indexer reports error if path elements are not supplied

[machawk1]

ipwb index /some/archive.warc -o myindex.cdxj

reports [Errno 2] No such file or directory: ''.

The remedy is to supply the relative path indicator to the file (./myindex.cdxj instead) but in other places, like ipwb replay myindex.cdxj, this relative path is not necessary.

We ought to

• Be consistent with whether we require the path element in indexer and replay
• Allow files to be specified for output from the indexer with an assumed path (foo.cdxj specified implies ./foo.cdxj)