Support scrolling beyond last line as an option.

This is an implementation of scrolling beyond last line based on setting padding-bottom for CodeMirror-lines element as was suggested in https://groups.google.com/forum/#!searchin/codemirror/CodeMirror-lines/codemirror/oAbm-FQtKv4/im8xpwysFhsJ

Padding bottom is always added if there is more than one line in the document.
Adding scroll in case of only one line seems awkward and the separate code path is needed to workaround padding-top set for the CodeMirror-lines element.