Support (and tests) for tuples with nested styles

In some cases, it would be nice if we could "nest" styles by passing tuples, like so:

```python
# creates a red phrase with some bold words in the middle:
t.red('This is ', t.bold('extremely important'), ' information!')

# deeper nesting of styles (green, then bold, then underline, and back out)
t.green('foo', t.bold('bar', t.underline('baz'), 'herp'), 'derp')
```

I've added tests with and without TTY, and so far it seems robust. Thoughts?