Added support for `cwd` option -- , ...

If its found amongst provided options, we prepend it to the file that gets sent to rimraf.

RE: https://github.com/sindresorhus/del/issues/4

Note I was not able to get tests in del repository running(via node test) at all. Its something really basic with the way tests are configured or dependencies are loaded, but due to that I wasnt able to add a fairly simple test case.

```
/Users/vlad/Projects/Javascript/del/test.js:6
beforeEach(function () {
^
ReferenceError: beforeEach is not defined
    at Object.<anonymous> (/Users/vlad/Projects/Javascript/del/test.js:6:1)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3

```