Properly support multiple mime outputs

[rapgenic]

We need to properly insert in the responses from the kernel multiple mime types, and at least text/plain, in order to support every interface (e.g. jupyter console, whch is terminal based and only supports text/plain outputs).

Native types

Support for these has alredy been implemented, see 45b636d.

Custom class based types

E.g. @tf and @sym. Enhanced output is provided by overriding the display function. It does not seem to be possible however to call the base function provided by those class to get a textual output. A possibility, although ugly, is to copy and paste the default function alongside the new one, to generate both outputs in the new one.

The best solution would be having package authors using on their own the xoctave functions to identify when running in a kernel and to display properly their output. This however requires a stable API (#8).

Original issue:

With console mode you mean jupyter console?
I tried to run some cells there and didn't work for me. I wonder if I did something wrong?
I just built it normally, do you know if there's the necessity of having some other stuff installed?

Originally posted by @marimeireles in https://github.com/rapgenic/xeus-octave/issues/5#issuecomment-856448763

Possibly related to mime outputs in the responses from the server. Need to check if the text/plain is always included.

[rapgenic]

@marimeireles I have updated the output infrastructure, and now jupyter console should mostly work.

See the updated issue text for more information.