Refactor s:add function

Hi, I made some changes:

The command `Plug` specifies that at least one argument must be given (`-nargs=+`) so we can explicit name the parameter it in the `s:add` function and simplify the parameter handling. Overall I wanted to extract the building of the spec for a plug from the adding of that spec in the `g:plugs` dictionary. I had fun doing it so If you do not agree with the changes there's no problem at all :)