Add pre_args

Add `--pre-args` argument, which specifies commands to be executed on the remote host prior to starting ipengine.

For example, I am using Theano on a GPU cluster and I want to avoid having different processes compete for the same graph compilation directory (since the locking mechanism only allows one compilation at a time), so in my PBS script I have:
```
cd $PBS_O_WORKDIR
id=`uuidgen`
pre_args='export THEANO_FLAGS="$THEANO_FLAGS,compiledir_format=tmp`uuidgen`"'
/home/kearnes/git/mixtape/scripts/pbsipcluster --daemonize -id $id -pre "$pre_args"
```
Which sets the Theano compile directory to a unique folder for each ipengine process.