Rename loop variable used for additional plugins

The variable `plugin` is used in two places, first when iterating over
`g:spacevim_plugins` and again when iterating over
`g:dotspacevim_additional_plugins`. On certain versions of vim a
"Variable type mismatch" occurs when attempting to iterate over
`g:dotspacevim_additional_plugins`. This appears to be because
`g:spacevim_plugins` contains dictionaries and
`g:dotspacevim_additional_plugins` contains strings.

Giving the loop variable a different name solves the problem.

Author: mikem

autoload/spacevim.vim

@@ -97,8 +97,8 @@ function! spacevim#bootstrap() abort
       endfor
     endfor
     if exists('g:dotspacevim_additional_plugins')
-      for plugin in g:dotspacevim_additional_plugins
-        Plug plugin
+      for additional_plugin in g:dotspacevim_additional_plugins
+        Plug additional_plugin
       endfor
     endif
     call plug#end()