Load completion support for zsh automatically; and tear it down appropriately in `__rvm_unload`.

I'm not sure I'm loading this in the right place; but I picked this place because this is where we used to tweak `MANPATH` (cf. 1f68d64)

Also, we may be able to simplify the nested if statement; this is the only way I found to know whether that work had already been done without getting weird error messages like `condition expected: "$rvm_path/scripts/zsh/Completion"`

Closes #1836.