shorthand helpers for fetching js or css

All use cases I've found in my apps right now needs either the JS files _or_ the CSS files, not both at the same time. So I tried to create a couple of shorthand helpers, so we don't need `{ js: true }` and `{ css: true }`. I think the api looks a little less noisy:

```javascript
var assets = require("gulp-assets");

gulp.src("./src/*.html")
	.pipe(assets.js())
	.pipe(gulp.dest("./dist"));
```