#897 optparse-applicative command line parser

Rewrote the command line parser using optparse-applicative.

* `--client` will need double quotes if you want to pass it several arguments
* `--exec` will accept arguments and `--execute` will default to "Main.main" (see: https://github.com/pcapriotti/optparse-applicative/issues/53)
* `--cpu` and `--target` does not need an `=` anymore
* `--help` generated by the library now looks like this: https://mediacru.sh/26ngg_Ft42u6

I did not find a way to add several options with one flag (as `-o` do for example), so I added a small utility function that add them afterwards (`preProcOpts` that is).

Any comments/suggestions would be appreciated.

#897