Fix dracula

[eleanor-clifford]

The dracula theme required variables defined in another file. With these changes the variables are defined properly and the theme is fixed.

Fixes #178

[stefanbuettner]

This fix works for me. Thanks

[benknoble]

dracula/vim maintainer here; another option would be to actually use vim8's packs. This repo could be a "package", and each colorscheme could have its own directory (git submodules, if you like that?).

At any rate, better would be to keep all the files from a particular colorscheme… as this issue + PR indicates, things can break when you ignore certain files.

[eleanor-clifford]

I agree, and using submodules would also make it easier to update the themes in this repository as they change upstream. @flazz seems pretty inactive though, I don't think this repo will be updated any time soon.

[EvitanRelta]

I've made a fork with a few colorschemes (including dracula) updated by via submodules and a script to add the /autoload files into the /colors/[name].vim file:
https://github.com/EvitanRelta/vim-colorschemes

Hopefully it'd be easier to maintain.