Add a video extension when user doesn't add one

[JoelMon]

When the user uses the -o flag, they have to add a video container extension otherwise the program crashes without any indication of what went wrong.

Example

$ scriptx -f nwt_01_Ge_ASL_10_r480P.m4v -v 20 -o Gen10-20
thread 'main' panicked at 'ffmpeg's exit status was FAILURE', /home/joel/.cargo/registry/src/github.com-1ecc6299db9ec823/scriptx-0.2.1/src/ff.rs:191:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Solution

1. If the user doesn't add an extension, add one by default
2. Improve error handling to be more informative