* Add `formatWithCursor` API with `cursorOffset` option
This addresses https://github.com/prettier/prettier/issues/93 by
adding a new option, `cursorOffset`, that tells prettier to determine
the location of the cursor after the code has been formatted. This is
accessible through the API via a new function, `formatWithCursor`, which
returns a `{formatted: string, cursorOffset: ?number}`.
Here's a usage example:
```js
require("prettier").formatWithCursor(" 1", { cursorOffset: 2 });
// -> { formatted: '1;\n', cursorOffset: 1 }
```
* Add `--cursor-offset` CLI option
It will print out the offset instead of the formatted output. This
makes it easier to test. For example:
echo ' 1' | prettier --stdin --cursor-offset 2
# prints 1
* Add basic test of cursor translation
* Document `cursorOffset` option and `formatWithCursor()`
* Print translated cursor offset to stderr when --cursor-offset is given
This lets us continue to print the formatted code, while also
communicating the updated cursor position.
See https://github.com/prettier/prettier/pull/1637#discussion_r119735496
* doc-print cursor placeholder in comments.printComments()
See https://github.com/prettier/prettier/pull/1637#discussion_r119735149
* Compare array index to -1 instead of >= 0 to determine element presence
See https://github.com/prettier/prettier/pull/1637#discussion_r119736623
* Return {formatted, cursor} from printDocToString() instead of mutating options
See https://github.com/prettier/prettier/pull/1637#discussion_r119737354
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