ENH: write Styler rendered output to file
I was answering a question on conditional formatting of html on [SO](http://stackoverflow.com/questions/37638402/pandas-html-output-conditional-formatting-highlight-cell-if-value-in-range/37647100?noredirect=1#comment62774932_37647100).

So the OP was using `to_html` but wanted conditional formatting. Therefore, I pointed to `Styler` to do this more easily than using the `formatters` argument in `to_html`, but when you have a `Styler` object, there is no longer a `to_html` method to write the result to a html file.

Of course you can write the result of `render` manually to a file, but would this ability be a desired enhancement?
- Maybe we can add a `to_file` method to `Styler`? (`to_html` seems a bit specific, if we also want to use the Styler for other formats such as latex in the future)
- Alternatively, we could add a `style` option in `to_html` where you can pass a Styler object?

Related to #11610

@TomAugspurger 
