runtime: Add vimscript support for external plugins

External plugins(a.k.a msgpack-rpc plugins) are now supported through a
library of vimscript functions that deals with:

- Associating plugin host names(eg: python, ruby, go) with channel ids
- Registration of external plugins
- Definition of commands, autocmds and functions lazily implemented over
  msgpack-rpc

runtime/plugin/external_plugins.vim

@@ -0,0 +1,5 @@
+if exists('loaded_external_plugins') || &cp
+  finish
+endif
+let loaded_external_plugins = 1
+call rpc#host#LoadExternalPlugins()