Add the 'raw' option to SSH shell initialization

[sourque]

This permits the use of the SSH shell as an arbitrary TCP tunnel by bypassing the startup environmental checks.

[sourque]

The reasoning behind this change is to allow pwntools to be used in non-standard SSH environments. For example, if SSH is configured to run a target binary with ForceCommand, or their default shell is changed, then the environment probing done by pwntools by default will cause the session to hang indefinitely.

This fails to initialize a session:

    p = ssh(user="bewilderment", host="bewilderment.hostbin.org",
            password="bewilderment").shell()

With this patch, the following now works as expected:

    p = ssh(user="bewilderment", host="bewilderment.hostbin.org",
            password="bewilderment", raw=True).shell()