Removed submodule plugins
Added plugins submodule

I actually added the submodule and committed, then did a hard reset
because I thought I did it incorrectly. However, I realized I did
do it correctly and was able to bring it back with --force. Then I
went into the plugins directory and did git submodule update --init
 --recursive which pulled in the ipythonnb submodule that I added
to the plugins through my own branch.