Adds support for nested chalk expressions.

green(a + blue(b) + c) will now look the same as green(a) +
blue(b) + green(c), as expected. In the previous implementation the
output would have been green(a) + blue(b) + c, because the reset code of
the second expression would close all expressions around it as well.

Now this reset code is replaced by a start code of the outer lying
expression, both stopping the inner and re-starting the outer.

For example:

![screen shot 2014-06-21 at 06 21 53](https://cloud.githubusercontent.com/assets/3891755/3347835/d235d92a-f8fc-11e3-9151-938c3d9a4c32.png)

Where previously everything after the inner green text would have been white.