Add Fish Support

[Spikeophant]

Hello,

I'm a fish shell user, and I have done a fork to make i2ssh work with fish, but it would be nice if it checked your shell, and if fish, did something different. Fish doesn't support &&; that's the main change. Tonight I'm going to do some additional work to add in a test to check your shell, and if it's fish, then run a different base cmd.

Dustin

[Spikeophant]

I added in a check, I'm not sure if it's elegant enough, however I check the $SHELL variable, and if it's fish, pass only @pane.cmd as there is no history set like bash, else do what was already being done.

[mbruggmann]

Feel free to open a PR. Happy to merge that as long as things still work in zsh (which is what I'm using).