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 tarjoilija#7

zgen.zsh

@@ -82,12 +82,28 @@ zgen-save() {
  rm "${ZGEN_INIT}"
  fi
 
+ echo "#" >> "${ZGEN_INIT}"
+ echo "# Generated by zgen." >> "${ZGEN_INIT}"
+ echo "# This file will be overwritten the next time you run zgen save" >> "${ZGEN_INIT}"
+ echo "#" >> "${ZGEN_INIT}"
  for file in "${ZGEN_LOADED[@]}"; do
  echo "-zgen-source \"$file\"" >> "${ZGEN_INIT}"
  done
 
  # Set up fpath
+ echo >> "${ZGEN_INIT}"
+ echo "#" >> "${ZGEN_INIT}"
+ echo "# Add our plugins and completions to fpath">> "${ZGEN_INIT}"
+ echo "#" >> "${ZGEN_INIT}"
  echo "fpath=(\$fpath $ZGEN_COMPLETIONS )" >> "${ZGEN_INIT}"
+
+ # Reset
+ echo >> "${ZGEN_INIT}"
+ echo "#" >> "${ZGEN_INIT}"
+ echo "# Clear ZGEN_LOADED and ZGEN_COMPLETIONS per #7">> "${ZGEN_INIT}"
+ echo "#" >> "${ZGEN_INIT}"
+ echo "ZGEN_LOADED=()" >> "${ZGEN_INIT}"
+ echo "ZGEN_COMPLETIONS=()" >> "${ZGEN_INIT}"
 }
 
 zgen-completions() {