Add "exclude" command

[lalaithion]

Issue #61 describes an idea for an exclude command. I needed the functionality of this command, so I added it. The code for it is largely based on the join command.

I also formatted the repository using cargo fmt.

[BurntSushi]

Please don't run rustfmt. I want to eventually switch to it, but doing so would make all extant PRs difficult to merge. Including rustfmt in this PR decreases its chances of me merging it.

[lalaithion]

Undid rstfmt.

[rodion-k]

I have tested it and find that rows are not excluded if they have empty fields. For example:
file1.csv

id,name
1,test1
2,test2
3,

file2.csv

id,name
1,test1
2,test2updated
3,

xsv exclude - file1.csv - file2.csv

id,name
2,test2updated
3,

Row 3 is not excluded.