Allow nested FormattingString

[jquast]

@jimallman brings us this convenience of allowing existing unicode strings to be joined as a call parameter to a formatting string result, allowing nestation:

>>> t.red('This is ', t.bold('extremely'), ' dangerous!')

This was rejected upstream as erikrose#45 His words,

# 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')

[jquast]

Merged by squash for next release