Add new option allowMultiParagraphSelection

If set allowMultiParagraphSelection to false, the toolbar will not
appear if you select several paragraphs or block
quotes. Same comportment has on medium.com
Default value is true

+ Testing Updated.
+ Demo Multiparagraph.html Added.
+ Demo CSS Updated.
Merge pull request #75 from devalnor/master

Add new option allowMultiParagraphSelection