disable I2P plugins

Plugins for I2P--without using persistence--don't make much sense.
A default of 'disable' here may also eliminate a possible attack vector.

I know of some Tails users that use I2P-Bote, a mail-like application
available as an I2P plugin, and they manually enable persistence for
I2P. A user that is able to manually enable persistence can easily
re-enable plugin support, if desired.