增加编码转换功能

[zhenglinj]

改变Buffer内容编码。比如，Windows平台常用 GBK 编码，转换为 UTF8 显示
iconv -f gb2312 -t utf-8 content.txt

[gucong]

I only work with UTF8. "cconv" can easily works with gbk or big5. Please adjust the "UTF8" in chinese-conv-cconv-command and chinese-conv-cconv-alist. Please see cconv -l. However, “opencc" need separate pre-/post-processing with iconv.

In the end, we need a systematic way of charset detection if we want to support non-UTF8. Pull request is welcome.