Feature request: ability to exclude topics

[Rayman]

With #8 we now have a way to snapshot all topics. It would be really nice to be able to exclude certain topics from this list (because they are too big/not needed). I've thought of two ways to implement this.

Option 1: Exclude regex

rosbag record has several options to filter which topics to record. We could also add these to the cli and ros parameters

Usage: rosbag record TOPIC1 [TOPIC2 TOPIC3 ...]

Options:
  -a, --all             record all topics
  -e, --regex           match topics using regular expressions
  -x EXCLUDE_REGEX, --exclude=EXCLUDE_REGEX
                        exclude topics matching the follow regular expression
                        (subtracts from -a or regex)

Option2: fix count

I've tried to exclude topics by changing the maximum message count to 0. However 0 is a special value that means "inherit from default options". We could change the special value to another value (-2?) so that 0 can be used to exclude topics.

<rosparam>
    topics:
    - /topic_name: {count: 0}
</rosparam>

I've also tried limiting the memory of topics to 1 byte, that works, but the topics are still subscribed, so I would like a better solution.

What do you think of this proposal?

[JanezCim]

I'd also welcome a way to exclude topics from being recorded. This feature has my vote. Thanks!