Add hidden cell property and rudimentary column support

@tealeg This adds a Hidden property to cells and basic column parsing. In a better implementation, the Hidden property would go on the `Column` and `Row` structs as they are in the XML document, but until we have better support for columns, I have opted to add the property at the `Cell` level. Obviously, this can be improved upon in the future.

```bash
➜  xlsx git:(hidden_cells) ✗ go test
OK: 72 passed
PASS
ok  	github.com/trinchan/xlsx	0.241s
```