Extract specific columns

[ettom]

Having the ability to extract specific columns directly via command line options would greatly improve the UX. Using cut -d' ' works fine until there are string columns which can possibly contain spaces. It's possible to work around this by replacing unescaped spaces with placeholders, but this is already pretty hacky. It would be very nice if you could just give sonnerie an argument like -f 1 to only output the first column.

[njaard]

@ettom I implemented it in the select-columns branch. Please give it a review before I merge it :)

Also, I'm considering a ND JSON output format which could be parsed with jq (at a huge performance penalty). Maybe each row would simply be a json array.

[ettom]

Tested, reviewed. Other than the very minor comment, LGTM!