Use `$PS4` debugging prompt to generate timestamps, instead of GNU `date`.
This also keeps timestamps and respective command together, something
that `paste` could not guarantee given some commands may have multiple lines.
The prompt timestamp has a precision of 6 decimals. The `printf "%12.9f\n"`
instruction with 9 decimals was kept to keep producing "the **SAME** output".
This could be replaced to `printf "%12.6f\n"` to make the output
smaller.
Also, only the first line of commands with multiple lines are saved, to
keep the output format backwards-compatible.
Closes#84
