add `join` function

[luyuhuang]

When I join multiple strings, I don't want to write 'aaa' .. '_' .. 'bbb' .. '_' .. 123, which is too long; I don't want to use table.concat either since construct a table is costly. So I add the join function, which provides a python-style way to join strings, just like ('_'):join('aaa', 'bbb', 123), as long as you did string.join = stringy.join.