Add Context.sudo_scope context manager

[Cc618]

As requested in #778, implemented a context manager that allows running multiple commands while changing user.

Usage

with c.sudo_scope():
    c.run("echo toto")
    c.run("whoami")  # root

# Supports `Context.sudo` options
with c.sudo_scope(user='hello'):
    c.run("whoami")  # hello

Implementation details

As prefix, Context.sudo_scope pushes its arguments to a stack Context.command_sudos.
When creating a command, the arguments from the nearest sudo_scope are used to create the command as Context.sudo does.