Apply `strings` to grep "ONLY printable characters" in files.

Some issue exist in ag utility on is_binary() function.
e.g., *.pdf files may considered as a non-binary file, and resulting in
dump "non-printable characters" to stdout.

After that pipe the result to stdin of `grep`, which will lead to a
binary data input, and grep will find 0 results matched.

When grep "-I" is not used, you're expecting to see this
Binary file (standard input) matches

However when grep "-I" is in use, ZERO matched result bumped.
pbcopy-nc: Copy (STDIN) content to tmux paste buffer simultaneously.