Add a username/hostname section to the prompt

[thenktor]

When connecting to remote hosts via ssh or when entering containers (Linux LXC, FreeBSD jails, Solaris zones) it is nice to see where you are working now. Therefore I've added a username/hostname section to the beginning of the prompt.

It always show username@hostname for remote/container prompts. You can set activate hydro_always_show_user to always show the username. Useful when using su to change user on local machines.

Fix for #50

[wunki]

I would love to see this merged, I am looking for similar functionality.

[auscompgeek]

Instead of performing all these checks on every prompt, given these parts of the environment wouldn't change during a given shell session, should this be cached?

I could also imagine there's scenarios where someone may wish to show the hostname, but this currently won't allow doing so on custom conditions.

[thenktor]

To be honest: I'm using the default fish prompt on all servers where one of these checks would turn on the show_hostname 😄 Probably a simple option hydro_always_show_hostname would be just as good, too. I cannot remember why I've added the automatic checks. Probably just because it was possible.