useTabs option works only partially

When running the following code:

``` js
var recast = require('recast');

var ast = recast.parse(
'while (true) {\n' +
'\tx;\n' +
'}'
, {useTabs: true});

// Add one line inside WHILE loop
ast.program.body[0].body.body.push({
'type': 'ExpressionStatement',
'expression': {
'type': 'Identifier',
'name': 'y'
}
});

console.log(recast.print(ast, {useTabs: true}).code.replace(/\t/g, ""));
```

I'd expect to get:

```
while (true) {
x;
y;
}
```

But instead Recast gives me:

```
while (true) {
x;
y;
}
```

NOTE: There are no docs to tell me whether I should pass `useTabs` option to `recast.parse()` or `recast.print()`, it seems that it's only needed for the latter, but just in case I'm passing it to both.

Start by running the provided JavaScript reproduction and compare the output from recast.parse and recast.print with useTabs enabled. Trace how indentation is preserved for existing lines versus newly inserted nodes; done means both x and y use tabs, matching the expected output, and the option behavior is documented.

Written by the indexing model from the issue text.