Support for handling the events (display on stdout, write to a file) after they are processed

[atenart]

Once an event is retrieved and processed we can provide it to the user. No post-processing is done at this point as we need all events for this and such things will be done offline.

Things to consider:

• Output the events to stdout, without (much) processing. Options are to use raw json, or a slightly better format for reading.
• The output might be controlled a little:
  • Select the collectors events are coming from.
  • Select specific fields to display. Here an option might be to have a formatting cmdline option such as --format "{timestamp} ksym: {ksym}"; or simpler options such as --show-field timestamp,ksym. We shouldn't support both though (for maintainability reasons).
• Write the raw json output to a file for later use. This will be required by post-processing commands later on.

For an initial support only a raw output to stdout might be possible to support. That is fine, if so please split this issue.

This depends on #8.

[atenart]

With #53 events are being printed on stdout. Writing them to a file isn't required for the PoC as we won't support post-processing. We also shouldn't have too many surprises when doing so as we already discussed different approaches for importing events (from json). Thus we don't need to work on this right away.

For the above reasons, delaying this to after the PoC.

[atenart]

In order to properly output the event, while they are being received, we should implement a Display trait and not repply on the json formatting. This will allow displaying nice types such as tcp flags:

• In json as a number (a bitfield).
• While displaying with proper names (ack, fin, etc.).

[atenart]

Fixed in #67. Will open a dedicated issue for future improvements.