Fix path on ftp

 `eachAsync ` was iterating on files already but not on different sources in `el.src`, only `src[0]` was uploaded so I couldn't take advantage of a declaration like `"src/*"`. The first file was uploaded in destination folder replicating the path from project root. I added the possibility to replicate the path from a chosen directory only (like the your).

for example previously with following config
```
files: {
    'public_html': 'src/*'
}
```
```finalRemotePath = path.join('/', el.dest, el.src[0])```
would have moved ```src/fixture.txt``` to ```/public_html/src/fixture.txt```

```
upload: {
    files: [{
	cwd: 'src',
	src: ['**/*'],
	dest: 'public_html'
    }]
}
```
will move ```src/fixture.txt``` to ```/public_html/fixture.txt```,
so you can choose with ```cwd``` from which folder start the tree replication.