String#trim(chars)

[Cyril Auburtin]

Like some other languages (ex: python .strip, postgresql trim, ..) I think
it would really useful that `String.prototype.trim` accept a string
argument representing the characters to trim (or possible a regex).

And keep it's default behavior if no argument is passed

```js
'\t Test \n\n'.trim(' \t\n') == 'Test'
```
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