Allow sending multiple lines per event

[vykintast]

Currently, LogentriesSink removes newline characters from events formatted as multi-line string before sending them to Logentries. This is a fine default behaviour, however, in some cases preserving multi-line formatting is preferable to preserving one-to-one mapping between Serilog log events and Logentries entries.

It would be nice to have a parameter which allows sending multiple lines per one event.

[vykintast]

As you can see above, I've opened a pull request that addresses this issue.
If some other approach is better, I'm open to suggestions.