Convert and check function arguments
This function is used for all variants of
rcmd. An argument
name is only used to refer to one kind of object, to make this possible.
List of options.
The benefit of having a single
options object is to avoid passing
around a lot of arguments all the time.
The benefit of making this object internal (i.e. that the
functions have multiple arguments instead of a single
is that documentation and usage is more user friendly (e.g. command-
completion works in the editor.