Add to_iruby_mimebundle support

[mrkn]

Introduce to_iruby_mimebundle method support like IPython's _repr_mimebundle.

The new display algorithm is below.

If the object to be displayed can respond to to_iruby_mimebundle, IRuby first tries to format the object by this method.
Then, IRuby tries to format the object by the renderers registered in the registry into MIME-types that are not formatted by to_iruby_mimebundle.
Moreover, IRuby tries to format the object by to_xxx method for the MIME-types that have not been formatted in the following list.

• text/html by to_html method
• text/markdown by to_markdown method
• image/svg+xml by to_svg method
• image/png by to_png method
• application/pdf by to_pdf method
• image/jpeg by to_jpeg method
• text/latex by to_latex or to_tex method
• application/javascript by to_javascript method

If none of MIME-type are formatted at this time, IRuby tries to format the object by to_iruby method.

Finally, IRuby generates the text/plain format by inspect method if text/plain format is not generated yet.

[mrkn]

CI fails due to network errors. I tried to re-run after some hours.

[mrkn]

CI fails due to network errors. I tried to re-run after some hours.

It was not due to network errors, but my fault by missing apt update before apt install.