feat(csvcut): Make -C ignore unknown columns

[lamdevhs]

Fixes #1280. It's my first PR, do tell me if I've done something wrong or if something is missing. :)

[coveralls]

coverage: 90.722% (+0.3%) from 90.435%
when pulling 13cf6a3 on lamdevhs:master
into d9ee911 on wireservice:master.

[coveralls]

coverage: 90.722% (+0.3%) from 90.435%
when pulling 13cf6a3 on lamdevhs:master
into d9ee911 on wireservice:master.

[coveralls]

coverage: 90.722% (+0.3%) from 90.435%
when pulling 13cf6a3 on lamdevhs:master
into d9ee911 on wireservice:master.

[jpmckinney]

Thank you!

[lamdevhs]

No, thank you! :) Sorry I got a few things wrong with the linting, versioning, etc. I agree with you that calling it a 'fix' is better, in fact that was what I'd have done on my own but I wasn't sure if you considered the previous behavior buggy or not. ^^"