Improve tlog writer: log when arm

[joaoantoniocardoso]

Expected behavior:

1. The tlogwriter driver now accepts a when argument, which can be either always or while_arm.
2. The default value for this argument is always to match the same behavior from mavlink-router, which is only configurable via conf-file, as described here. For this reason, this patch introduces a major behavior change.
3. If the passed path is a folder that doesn't exist, it will try to create the entire path.
4. When operating as while_arm, it will start a new file every time the vehicle changes from disarmed to armed, following the same file name format as mavlink-router. On the other way around (when the vehicle changed from disarmed to armed), it closes the file.
5. When operating as while_arm, the system_id and component_id from which it monitors the heartbeats can be configured from endpoint arguments, falling back to the same behavior as mavlink-router: it gets the system ID from the first heartbeat sent by an autopilot (component id 1).

• Tested locally
• Tested on the vehicle w/ Navigator

Closes #54.