Script to automate weight generation

[liamaharon]

Now that the weight generation has been documented (#127), a script should be written to automate the process.

Should take inputs like:

1. Set of runtimes & pallets/extrinsics to bench
2. SSH credentials for a benchmark machine
3. Local output dir

and pallet-by-pallet exec the benchmarks on the remote machine and write the results to the specified local directory.

the command should by default 'pick up from where it left off' if it was terminated early, e.g. be tolerant to stopping/starting and not repeat benchmarks that have already been done.