Field minSize and maxSize

To filter useless files with respect to my application, a lower file size limit would be useful.

Example: I will never receive files smaller than 1MB.

Reasoning: This would be cleaner, than checking the file size manually in `onFileUploadComplete`.

Proposal (backwards compatible):

```
limits: {
fileSize: [LOWER-LIMIT, UPPER-LIMIT]
}

limits: {
fileSize: UPPER-LIMIT
}
```