Added type 'custom' to error_bars on charts

Hey John -

For my project I needed Excel's 'custom' type for chart error bars. I've included a minor change to accomodate that.

Example usage is as follows:
```
        chart.add_series({
            'categories':   '=A2:A8'),
            'values':       '=C2:C8',
            'y_error_bars': {
                'type':      'custom',
                'value':     ('=B2:B8', '=D2:D8'),
                'direction': 'both'}})
```
In this case my cell composition looks like this:

|        | First Quartile          | Median  | Third Quartile |
| ----------------- |:-------------:|:-------------:|:-------------:|
| Red     | 12 | 15 | 25 |
| Blue    | 10 | 45 | 60 |
| etc      |     |      |     |

Please let me know what changes/tests you would like in order to be merge-ready. Thanks!