Allow users to opt out of the default kernel.

[vchuravy]

Shared Jupyter installations might want to be more selective.

[stevengj]

Should it be stored in the prefsfile so that this setting is remembered on updates (which needs to get migrated to the new Pkg-based system)? In which case it should be IJULIA_INSTALLKERNEL=no or ``IJULIA_INSTALLKERNEL=yes` (the default), maybe?

[vchuravy]

For the use case I need this, it's easy to just set this in the script that brings up Julia + IJulia, and I am probably fine with users manually upgrading IJulia and installing a new kernelspec.

Preferences.jl only works on Julia 1.6.

[stevengj]

Fine; can you add a note to the README?

[stevengj]

I added a note to the documentation.