Implement mlr uniq -x

[johnkerl]

Resolves #1456.

Examples:

$ mlr --c2p cat docs/src/example.csv
color  shape    flag  k  index quantity rate
yellow triangle true  1  11    43.6498  9.8870
red    square   true  2  15    79.2778  0.0130
red    circle   true  3  16    13.8103  2.9010
red    square   false 4  48    77.5542  7.4670
purple triangle false 5  51    81.2290  8.5910
red    square   false 6  64    77.1991  9.5310
purple triangle false 7  65    80.1405  5.8240
yellow circle   true  8  73    63.9785  4.2370
yellow circle   true  9  87    63.5058  8.3350
purple square   false 10 91    72.3735  8.2430

$ mlr --c2p uniq -g color,shape docs/src/example.csv
color  shape
yellow triangle
red    square
red    circle
purple triangle
yellow circle
purple square

$ mlr --c2p uniq -c -g color,shape docs/src/example.csv
color  shape    count
yellow triangle 1
red    square   3
red    circle   1
purple triangle 2
yellow circle   2
purple square   1

$ mlr --c2p uniq -x flag,k,index,quantity,rate docs/src/example.csv
color  shape
yellow triangle
red    square
red    circle
purple triangle
yellow circle
purple square

$ mlr --c2p uniq -c -x flag,k,index,quantity,rate docs/src/example.csv
color  shape    count
yellow triangle 1
red    square   3
red    circle   1
purple triangle 2
yellow circle   2
purple square   1