Add comments for messages

### Is your feature request related to a problem? Please describe.
I am using this library to analyze can traces in several formats. Sometimes i would like to add some comments to can messages that are then printed inside an output trace file. This is supported for example by the asc file format.

### Describe the solution you'd like
The class `Message` should get an attribute `comment` by default (e.g. None by default)

### Describe alternatives you've considered
right now i am using some hack to write to the file object inside the writer classes but this is quite unstable when the library changes

### Additional context
I would like to implement this and add it to the reader and writer classes as well but first i would like to get some feedback on this feature request.