Backport [2/5]: xinterpreter improvments

[rapgenic]

This set of modifications backports some general improvements in the interpreter:

• The most important is surely the updated method for keeping the right search path for octave files, which no longer relies on overriding the addpath.m file but on a native C++ hook that is executed every time the path is changed and automatically fixes it as needed. (NB if you had already installed octave you'll need to remove the file in /usr/local/share/xeus-octave/addpath.m)
• General clean up and more comments in the code
• Help request presents multiple mimetypes if they're available

[AntoinePrv]

Thanks for making this changes!

[rapgenic]

@AntoinePrv sorry for pinging, I have fixed the xeus::register_interpreter as you suggested, and the fact that 5.3 protocol version comes from the cookiecutter is good enough for me to consider it "right".

There have been no other significant changes, so unless you need to review it again (just tell me in case, no need to do it in a short time :smile: ), I'll be merging this in the next few days.

Thanks!

[rapgenic]

Thank you for the changes, and don't worry, I didn't mean to put pressure to you...

I like better the get_protocol_version call, and to avoid doing the change again I'll leave the pointer instantiation as is, we can always change it later... it doesn't really make any difference at this stage.