Disable auto-start for multiplexers with SSH

I was playing with SSH and ended up on my own machine (don't ask) which is for me, the only one using the `auto-start` option (`tmux`/`screen`).

The problem is that I ended up with a multiplexer session inside another multiplexer session which is really confusing (even more in my case, where the two sessions were actually the same...).

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In general having a multiplexer inside another multiplexer is just plain bad, but when it comes to remote connections there is no way to tell if the user is already in a multiplexer or not.

I guess the best action against that, is disabling `auto-start` for remote connections by default.
Then the user will be able to start the multiplexer manually if he wants to.

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I understand that some people only use a multiplexer on the server side (so, there is no way that the user could already be in a multiplexer), and want `auto-start` to work properly with a remote access.

I added a new option for `screen` and `tmux` forcing `auto-start` to work with remote connections:

    zstyle ':prezto:module:tmux' remote 'yes'

This will allow auto-start for an SSH connection on a server.


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The documentation and `zpreztorc` have been upgraded