Add support for reading CSV from shell stdin via the `-` file input

[nickdos]

I have a shell script that tests the conversion of JSON to CSV that I'd like to use with csvlint but I don't want to force the script to save (and later delete) an actual file. Rather I'd like to pass the CSV in via stdin in the same way most command-line utilities work.

I tried:

cat file.csv | csvline -

but it throws an error file '-' does not exist.