Zero to one arguments for :w and :wq

Vim does not allow writing to two files at once with :w and :wq:

    :w test1.txt test2.txt
    E172: Only one file name allowed

It's therefore more accurate to use -nargs=? rather than -nargs=*.
Add :wa
:wqa only accepts zero to one arguments

Much the same as pull request #2:

<https://github.com/takac/vim-commandcaps/pull/2>
Prevent overloading, don't force commands
Check +user_commands feature is available

Required for :command to work
Update README.md with new commands fixed
Restore many-possible arguments for commands

I didn't know about ++opt
Merge pull request #2 from tejr/master

Prevent overloading, check for `user_commands` feature, add :wa