Add API to attach multiple transports/parsers to a link

[flixr]

Currently one normally uses the <transport>_check_and_parse functions to - well - check if there are new input chars on the device and if yes parse them using the transport.

It would be good to have the possibility to easily attach multiple transports/parsers to a link and pass the incoming data to all of them.

See also #10

[podhrmic]

Is this still relevant? @flixr

[podhrmic]

@gautierhattenberger @flixr is there still interest to do this on the pprzlink side rather than on the higher level application side (such as paparazzi)? Or can we close this?

[podhrmic]

Looks like this can be handled with user defined transports, see #70 for discussion.