Added a jshintignore option

This patch solves #1.

If a filename is given as `jshintignore` option then any directories or files specified will be skipped over.
For example, there are two files `foo/bar.js` and `foo/baz.js`, and the content of `.jshintignore` is

```
foo/bar.js
```

then following `jshint` task checks `foo/baz.js` but ignores `foo/bar.js`.

```javascript
jshint: {
  all: ["foo/*.js"],
  options: {
    jshintignore: ".jshintignore"
  }
}
```