Format string for blame_line timestamps

[mrjones2014]

Is your feature request related to a problem? Please describe.
I'd like the timestamp in the blame_line popup to use 12 hour (AM/PM) time instead of 24 hour time. This could be solved by allowing me to set a custom time format string.

Describe the solution you'd like
Add a config option to specify the timestamp format string.

Describe alternatives you've considered
Forking the project, I guess? I don't think there's any way to do this without code changes.

Additional context
24 hour time adds mental load (a small amount, but still some). I'd like to be able to see the time in 12 hour (AM/PM) time.

[lewis6991]

Duplicate of #291