Support for custom keyboard shortcuts

 Added support for keyboard shortcuts as a configuration option. Default values still present if not specified.

e.g. can now create an editor as follows:

var shortcuts = {
    "66": "bold",               // B
    "73": "italic",             // I
    "76": "createLink",         // L
    "78": "insertUnorderedList",// N
    "79": "insertOrderedList",  // O
    "85": "underline"           // U
};


var editor = new wysihtml5.Editor("body", { // id of textarea element
    toolbar:      "wysihtml5-toolbar", // id of toolbar element
    parserRules:  wysihtml5ParserRules, // defined in parser rules set
    useLineBreaks:  false,
    shortcuts: shortcuts
});

Added support for dialog commands from keyboard shortcuts, such as link.