join_separator support for the output log line

[bernii]

The plugin is generating a flat log line which is really hard to read as full stack trace in a single line is not really human readable.

This PR adds support for join_separator that allows the user to customize the character that is used when concatenating detected stack trace lines (for example join_separator "\n" can be used).

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[bernii]

I signed it!

[googlebot]

CLAs look good, thanks!

[thomasschickinger]

AFAICT your concern with the long log line is addressed by the pending PR
#10.

The fact that exceptions from text logs could end up on a single line was never intended. The mentioned PR makes sure that there is always a line separator in between. PTAL whether this addresses your requirements.

[bernii]

Yeah looks like #10 already solves it! I'm going to close this one then, thanks!