[RFC] Improve python API.

Add a utility function to make creating python api session more comfortable.
This was suggested at neovim/neovim#1373.

For example, instead of:
```
from neovim import socket_session, Nvim
session = tcp_session(address=<address>, port=<port>)
nvim = Nvim.from_session(session)
```
you can do now:
```
from neovim import attach
nvim = attach('tcp', address=<address>, port=<port>)
```
Also:
```
nvim = attach('socket', path=<path>)
nvim = attach('child', argv=<argv>)
nvim = attach('stdio')
```