Add --map-level to rewrite log levels

[hackedd]

This allows rewriting the levels in the log to values that are recognized by fblog. Particularly useful when logs contain numeric levels like those generated by Bunyan or syslog.

[bomgar]

The level_to_style function in template rs maps those levels to colors. I think the available options should match. Maybe just add the colors for the new levels trace and fatal. Maybe the supported levels should be constants somewhere 🤔 . Although I thought awhile ago that I should move the colors to the config and get rid of a fixed set.

Otherwise this looks good.

[bomgar]

I removed the default. It felt weird to include numbers of some arbitrary node library. I added the example to the readme.

[hackedd]

Fair enough!