Accept custom OptionPrefix as CLI argument

[mattshin]

I would like to be able to run kube-prompt with something like:

$ kube-prompt --prefix "foo>"

and then be given a prompt with prompt.OptionPrefix set to foo>.

This could be useful when running kube-prompt in different contexts to know which one you are in.

[mattshin]

I don't mind doing some work on this if you greenlight it as a feature you would be willing to add

[c-bata]

I close this (see #52 (comment))