ytt should support providing arguments via a file

This would benefit ytt users who want to template a variable list of files with a single repeated command.

ie. One could provide a file which indicates the `-f` arguments so that in a pipeline (such as argoCD) you could always provide the same command (maybe `ytt --arg-file file.yaml`) and the contents of the file could be updated to reflect the desired templating.

Further discussion could be useful to determine
1. Is this valuable for users?
1. The name of the flag

No implementation files or tests are identified in the issue. First review ytt's existing -f command-line argument handling and the unresolved discussion about whether file-provided arguments are valuable and what the flag should be called; done means an agreed interface that supports the described pipeline use case.

Written by the indexing model from the issue text.