Added privateKeyPath support

It is my understanding that: 

```js
privateKey: grunt.file.read('<%= config.passphrase %>'),
passphrase: '<%= config.passphrase %>'
```

is not possible (grunt doesn't understand template variable within it's functions). 

This is kind of needed when multiple users use the same Gruntfile.js. I decided to make a little patch which adds support to private key paths. I changed README.md to reflect this addition, but I did not modify neither Release History nor package.json

If you find this worthy addition please apply the patch and bump npm version. Thanks :)