打开js文件时，自动切换到js2-mode
(add-to-list 'auto-mode-alist '("\\.js\\'" . js2-mode))
其中“\\.js\\'”是正则表达式用来匹配js文件，
“\\.”匹配“.”，“\\'”匹配字符串结尾
注意：
http://www.emacswiki.org/emacs/AutoModeAlist
Note that \' matches the end of a string, where as $ matches the empty string before a newline. Thus, $ may lead to unexpected behavior when dealing with filenames containing newlines. (Should be pretty rare…)
