4chanquotes

IMPLEMENTS #2 

the new style will be available only if `multiline_quoting` converter is set to `true`.
It will be used if the first two lines of a quote start with `>`, and only the lines that start with `>` will be included in `[quote]` tags.

![4chanstyle](https://cloud.githubusercontent.com/assets/1429402/2896935/6bdf10ec-d575-11e3-8309-8ba4d420492b.PNG)