textarea在光标处插入指定内容：
function insertAtCaret(areaId, text) {
var txtarea = document.getElementById(areaId);
var scrollPos = txtarea.scrollTop;
var caretPos = txtarea.selectionStart;
var front = (txtarea.value).substring(0, caretPos);
var back = (txtarea.value).substring(txtarea.selectionEnd, txtarea.value.length);
txtarea.value = front + text + back;
caretPos = caretPos + text.length;
txtarea.selectionStart = caretPos;
txtarea.selectionEnd = caretPos;
txtarea.focus();
txtarea.scrollTop = scrollPos;
}
