added 'jvm_arg' option for jar-tool config in pants.ini

This does not solve issue 169, but it makes is less painful and easier to configure/diagnose.

Testing Done:
Added this to pants.ini:

[jar-tool]
jvm_args:['-Xmx257M']

I used '257' because its an unlikely value.

PANTS_DEV=1 ./pants goal ng-killall
PANTS_DEV=1 ./pants goal bundle <target>

ps auxww | grep 257M shows the NGServer process.

Bugs closed: 169

Reviewed at https://rbcommons.com/s/twitter/r/421/