Allow setting "missing value" string when using join.

[ghuls]

It would be nice that it would be possible to set a missing value string when using join, so it is possible to put for example "NA" instead of empty fields.

[ghuls]

Another option would be to add a --null option to fmt, so fmt can replace all empty fields with a specific string:

xsv join 2 test1.csv 2 test2.csv | xsv fmt --null 'NA'

If the -null option is also added to input, it would allow to specify a string which, if found will result in a NULL value (empty column) for xsv. This probably would solve: #38

[BurntSushi]

I think this is a straight out duplicate of #38, no? I think even the proposals you've given were already suggested in some form in #38.

[ghuls]

Yes, it is very similar to #38.