Fix dump could deal the encoding

Current version of `galileo dump` command cannot dump the utf-8 encoded pages like snowflake properly. (Of course utf-8 encoded CJK characters cannot be dumped to file.) This patch adds `--encoding` or `-e` options for `dump` command. By default it does nothing like original code. But if someone have trouble with dump due to utf-8 encoded characters then `--encoding=utf-8` option could help.

I've just tested under default SQLite database (as Galileo default setting) and it just works. And added document is very shoddy so please consider that. :)