Support getting a reasonable amount of log content across different log files

[sakuraehikaru]

While we have previousLogFileURL, most apps will probably have to implement a way to handle log content that might be spread across different files, especially if a file rotation was recent.

A simple version could just be always export the current and previous files together without care to the content size, and a more advanced version could be one that allows specifying the number of lines expected in the exported content.