Comments RTL support

Add an RTL (right-to-left text) parameter to the comments request allowing the application of RTL styles to the comments.
This is just a small part of the required change. In order to achieve full RTL support, an RTL stylesheet must be added server-side when the RTL parameter is present.
The required css file is attached to the corresponding issue [here](https://github.com/Automattic/jetpack/issues/215).