API: Ability to set cwd for .jshintignore lookup

In [Web Essentials](https://github.com/madskristensen/WebEssentials2013/), we have recently changed the way to interact with jshint and other node-based services: from CLI to the service API.

The [implementation is successful](https://github.com/madskristensen/WebEssentials2013/blob/43579ede606188a8e12b15de96e0c18b3384a41b/EditorExtensions/Resources/server/services/srv-jshint.js), except that we can't consume the ignore file implementation due to the reason that the current working directory can't be set via options (and `process.cwd()` is pointing to the location, where our web-server resides).

This PR let the implementer pass cwd via opts, so `findFile()` can start the search from custom (source-file) path, instead of `process.cwd()`.