Some proposed enhancements

[michaelblight]

Hello Alex - not sure if you're interested in other contributors, but I wanted a couple of options myself, so thought I'd give it a bash.

The changes I'm proposing are:

1. Allowing log level to be optional by including a "(none)" option.
2. Allowing msg.topic to be included in the log if present. This is set up in the log-config page when the log style is plain text. This defaults to unselected for backward compatibility.
3. Allowing the source (eg. msg.payload) to be optional. This is also set up in the log-config page when the log style is plain text. I think this should be backwardly compatible, because it's defaulting to selected.
4. If both topic and source are included, they're separated by a ":".

What do you think?

[michaelblight]

I've also added the ability to rotate the log on demand by setting msg.rotateNow to true. I'm doing this at midnight to get daily log files.