display elapsed time instead of remaining time in timestamp

[github-account1111]

Subject of the issue

I want [elapsed time]/[length] as opposed to -[remaining time]/[length].

Your environment

• version of pianobar 2020.04.20
• your Linux distribution and release version Windows 10
• ffmpeg/libav version and the flags it was compiled with (if you compiled yourself)
• your config file

Steps to reproduce

Play a song

Expected behaviour

[elapsed time]/[length] in the timestamp field.

Actual behaviour

[-remaining time]/[length] in the timestamp field.

[PromyLOPh]

Should be possible to extend BarMainPrintTime, there’s already a format string (format_msg_time).