Control the composing behaviour (splitting window/use current window)

[seblemaguer]

However, it does change a bit the composing behavior. With version 4.0 of the package, when I compose a message from the headers buffer, it splits the window. Same thing when I am reading a message. With this PR, it substitutes the contents. So there is not split, just a window to compose that replaced whatever was originally in the window.

Originally posted by @manueldeljesus in #207

[seblemaguer]

@manueldeljesus could you provide your configuration? It splits the window on my side