Add a --files option

[johnkerl]

The user would supply one file name per line like

cat filenames.txt

file0001.csv
file0002.csv
...
file8956.csv

Then they could run mlr --files filenames.txt ...

Originally posted by @derekmahar in #1421

[derekmahar]

Thank you!

[aborruso]

Thank you

[janxkoci]

Just curious, does it do the same thing as the following code?

mlr --mfrom $(cat filenames.txt) --

[johnkerl]

@janxkoci yes. With the exception that if filenames.txt is very long, putting them all on the command line via interpolation can result in an error from the shell saying the argument list is too long. Whereas mlr --files will step through filenames.txt successfully without hitting that limit.

[janxkoci]

I see, cool!