Improved HPC Support through cluster_config()

[shikokuchuo]

Closes #296.

Implements cluster_config() for full granular control over HPC job schedulers such as Slurm sbatch, SGE / Torque / PBS qsub and LSF bsub.

These systems expect a shell script to be executed with the full configuration options contained within, including the loading of environment modules etc., which would otherwise not be possible to pass in a single command line.

Rather than dynamically create an actual script on the filesystem, this implementation employs a Heredoc for more optimal programmatic control.

EDIT: rebasing after merging #298, now inherently supports array jobs just by adding a line #SBATCH --array= or equivalent in the options.

FYI @michaelmayer2