fix: Newlines in 'exif_call()'s 'args'

[trevorld]

• Fix by finding out which args has newlines, escape them, and prepend those lines with #[CSTR]
• The #[CSTR] newline feature in exiftool args files is documented here: https://exiftool.org/faq.html#Q21
• Note grepl("\n", "boo\nbar") is TRUE but grepl("\n", "boo\\nbar") is FALSE. In particular if a user of {exiftoolr} has already manually done this correction then it shouldn't happen a second time (and break things).

closes #14

[JoshOBrien]

@trevorld Thanks for this! I appreciate your help.