destDir: default "/", accept string, tests

Should be able to write
```js
catalog = concat( flatten(postPages), {...
```
instead of:
```js
catalog = concat( flatten(postPages, { destDir: '/' }), {...
```
Also, `destDir` is your only option param, so should be able to write:
```js
var vendor = from('vendor'),
	vendorCss = flatten( vendor('.css'), '/css' )
```
as shorthand for:
```js
var vendor = from('vendor'),
	vendorCss = flatten( vendor('.css'), { destDir: '/css' } )
```

I don't like submitting PR w/o tests and you didn't have any choosen test fw, so I put in simple test file.