Add terminal size fit for side by side view

Love your tool !

But look what messy hack I've put in my .gitconfig:

```cfg
[alias]
    cdiff = !cdiff -s -w$(echo "$(stty size | cut -d ' ' -f2) / 2 - 5" | bc )
```
And it's not even correct because I couldn't manage to use parenthesis in the gitconfig cmd expression :-)

I guess it would be better if supported by cdiff itself.

So here's a pull request that adds a terminal size auto-detection that is activated when using `-w0`.
Maybe you'll prefer an explicit switch for that feature ? Just tell me.