Adding a method for saving content

[DominicWrege]

It's not clear how can the user save a the writing text from that editor. And I didn't found any documentation which has a save method.

[miztroh-zz]

You can already use the value property for this. It was missing from the documentation because the current master branch of iron-component-page doesn't properly pull in behavior dependencies and generate docs for them (see PolymerElements/iron-component-page#39). I've updated my GitHub pages version of the documentation to the pending PR version of iron-components-page (see PolymerElements/iron-component-page#86). I also added documentation to my other two repos which are dependencies of this one, selection-mgr and undo-mgr. All methods, properties, etc. from those behaviors now appear in the documentation at https://miztroh.github.io/bower_components/wysiwyg-e/.