Warn about unmatched records in ndjson-join

[bianchimro]

I'm using ndjson-join to make 1 to 1 joins between two files and I'd like to check that after the operation each record of the LHS stream is present in the final recordset.

Counting the final records by piping into wc -l and comparing to the length of the recordset might be misleading as we could have 0 matches for some records and 2 or more matches for some other.

For my use case It would be great to have a command line switch enabling warnings about unmatched records in the LHS stream, for example --check-joins or something like that.

[mbostock]

Release 0.3.1 adds support for left, right and outer joins (#23). This can be used to check for missing joins by filtering for null after joining.