add grunt-cli

I was setting this up on a laptop I don't normally use for node, and ran
into an error that more or less equates to bash throwing a "command not
found" for grunt. Adding grunt-cli makes sure npm installs the grunt
command line binaries.

/cc @tkaufman @theotherzach -- I won't be able to make the thing tomorrow (at least the beginning), but maybe this commit will save someone from getting a silly error off the start :) Hope it helps.