Improve command line handling

Adding cfg options `hpx.commandline.aliasing` (control command line aliasing) and `hpx.commandline.allow_unknown` (allow unknown options to be passed through to hpx_main).

Remove `--version` and `--help` command line aliases.
Moved command line alias definitions from `[hpx.commandline]` to `[hpx.commandline.aliases]`.
Added example demonstrating this.