Final commit: direct users to the new plugins

* Removed all code, so it is obvious the plugin is no longer supported.
* README updated to guide users to the new plugins.
Fix list of configuration variables changes
Merge pull request #118 from vim-pandoc/final

[final commit] Becoming vim-pandoc-legacy

* Removed all code, so it is obvious the plugin is no longer supported.
* README updated to guide users to the new plugins.