Change order of arguments to internal `wd add` function

This enables us to detect empty warp points names such as "   " or ""
Force overwrite of $CONFIG file in `wd_remove`

If people set the NO_CLOBBER option in zsh, the write will fail. We can
force the overwrite by using `>!` instead.
Remove shifts

We don't know how many arguments the user supplied. Shifting a lot of
arguments that might not be there doesn't sit well with me.
Change help text for `wd ls` and `wd rm`

You cannot write `wd list` but you can however write `wd ls`. The help
text doesn't really makes this clear but rather references `wd --list`
and `wd --remove` instead of `wd ls` and `wd rm`.
Install a function instead of an alias

Function should be preferred to aliases in most cases
Prettify completion
[Fix?] Rename `wd remove` to `wd rm`

I think there has been, and maybe still are, some misunderstandings and
misalignments between the printed help and what arguments the program
actually accepts. This commit renames the accepted command `wd remove`
to `wd rm`.
Merge pull request #3 from malthejorgensen/master

Update completion, remove shifts, update help text (`wd ls`)