Add a "Highlight" filter

[elliotjo]

Similar to the Include/Exclude filters, it would be handy to have a "Highlight" filter that does not actually affect which messages are displayed, but instead highlights specific ones so that they can be easily seen when lots of messages are scrolling by.

A good question is, how should they be highlighted? We can change the color, but since the log levels are being colored now, we would probably want to make that customizable to ensure it would stand out. Alternatively, the background color could be changed, or the log text could be made bold or italic....