Add checks against the arguments passed to the executable

[mhchia]

What is wrong?

1. Arguments are not checked against the format. Per the feedback from Peering Issues - master branch #144 (comment), it will be better if we check if the given arguments are valid.
2. There is no hints for commands. E.g. Users cannot know what commands are supported through the CLI.

How can it be fixed?

1. Add checks, or see if low-hanging solutions are available.
2. Parse the RPC commands with flag