runtime: Add vimscript library for dealing with msgpack-rpc plugins
runtime: Implement python/clipboard providers with autoload scripts

Clipboard is implemented with platform-specific shell commands, and python is
implemented with the msgpack-rpc plugin (rpc#* functions). The script_host.py
file(legacy python-vim emulation plugin) was moved/adapted from the python
client repository.
runtime: Remove 'python_setup.vim'
test: Add tests for the rpc#define#* functions
test: Add tests for the python provider

These tests were adapted from the python-client repository