Get the current command's value

@param {String} command The command string which to check (eg. "formatBlock")
@return {String} The command value
@example
var currentBlockElement = commands.value("formatBlock");
function(command) {
    var obj     = wysihtml5.commands[command],
        method  = obj && obj.value;
    if (method) {
      return method.call(obj, this.composer, command);
    } else {
      try {
        // try/catch for buggy firefox
        return this.doc.queryCommandValue(command);
      } catch(e) {
        return null;
      }
    }
  }