Fixed issue with g:godown_autorun being undefined

[sbdchd]

Before:

Error detected while processing ~/.vim/plugged/godown-vim/ftplugin/markdown.vim:
line    2:
E121: Undefined variable: g:godown_autorun
E15: Invalid expression: g:godown_autorun
Press ENTER or type command to continue

After:

No issue

[davinche]

Thank you for the PR. However, there shouldn't be an issue with g:godown_autorun being undefined since it is defined: https://github.com/davinche/godown-vim/blob/master/plugin/godown.vim#L13-L15

How are you loading the plugin? Also, which version of vim are you using? I tested on vim8 and neovim 0.17 and both seems to work.

[sbdchd]

I have filetype detect trigger inside my config file so that I can reload the config without having to restart Vim; however, this results in godown not being able to find the variable g:godown_autorun.