Add string/replace, simplified version

This adds basically the same thing as #20, but with much simplified code and API. It uses the most straightforward way to simplify multiple `str.replace` calls.

From the docs:

```js
replace('foo bar', 'foo', 'test');                // "test bar"
replace('test 1 2', ['1', '2'], 'n');             // "test n n"
replace('test 1 2', ['1', '2'], ['one', 'two']);  // "test one two"
replace('123abc', [/\d/g, /[a-z]/g], ['0', '.']); // "000..."
```