Allow specifying args in a file.

Helpful for when command lines are otherwise too long.
A safer way of splitting file contents into lines.

Note that I don't use list(fp) or fp.splitlines() because
I don't want the trailing newlines in the strings.
Only sync-all if the user specified no args.

Previously if the user specified args that resolved to an empty list
of files, we'd still sync-all.
Create a decorator for file-glob args.
Merge pull request #10 from benjyw/argfiles

Allow specifying args in a file.