Add new binary util - mdb-exportjson

A new binary util for exporting mdb rows as json lines.
This is very useful for importing mdb data to mongo in combination with mongoimport.

The code is vastly similar to the mdb-export but there are enough dissimilarities and disagreement
about flag values that I thought a separate util is warranted.

I'm sending this pull request to see if you have interest in adding this to mdbtools' main line.
If this is interesting to you, I can easily fix some of the style differences.