Added some more output formats to examples/mavlogdump.py

Instead of only the proprietary output format, I've also added a JSON output and a CSV output format. The CSV format can be used with any character (like `,`) or tabs and can do a zero-order hold on the data making it easier to process. It also merges messages from the same timestep to ease processing. I use it for exporting certain messages from a MAVLink stream into a CSV file for MATLAB.