Added basic activate/deactivate, conda activate/deactivate/ls for fish shell

This is a basic take on fish-compatible `activate` and `deactivate` commands. This also adds `conda activate`, `conda deactivate` and `conda ls` commands for convenience.

Autocompletions and event handlers may follow later; the basics seem to work fine for now.

Usage:

```sh
. $ANACONDA_DIR/bin/conda.fish
```