Eliminate zombie plugins, completions and themes

We weren't claring `${ZGEN_COMPLETIONS}` and `${ZGEN_LOADED}` after
loading `${ZGEN_INIT}`. This made it impossible to delete a plugin -
every time I'd do `zgen save`, it would include them since they were
already in the arrays.

Closes #7
Simplify per @tarjoilija's feedback

Use source in `${ZGEN_INIT}` instead of -zgen-source. Fixes #7, with the
welcome side effect of being 11% faster.
Merge pull request #8 from unixorn/night-of-the-zombie-plugins

Eliminate zombie plugins, completions and themes