Update function genfromtxt: add keyword `nrows`

Add keyword `nrows` to specify the number of rows to read.
1. We should not use keywords `nrows` and `skip_footer` in the same time. 
2. If there's not enough lines (less than nrows) to read, an exception would raise.
I don't like this... I'd rather leave it as `None`, and later have the check be something like:

```
if nrows is not None and len(rows) >= nrows:
```