ucbdiaz2015

Results 2 issues of ucbdiaz2015

### Summary Want to be able to override the search method used for MultiSelect.Filterable. Downshift takes [onInputValueChange](https://github.com/downshift-js/downshift/tree/v1.31.16#oninputvaluechange) prop, but carbon sets its own method for that: https://github.com/carbon-design-system/carbon/blob/master/packages/react/src/components/MultiSelect/FilterableMultiSelect.js#L330 Adding custom method...

type: enhancement 💡
status: help wanted 👐
component: multi-select

### Expected Behavior Should call renderValue just as YAxis calls it renderValue: https://github.com/grommet/grommet/blob/master/src/js/components/DataChart/DataChart.js#L508 YAxis (correct): https://github.com/grommet/grommet/blob/master/src/js/components/DataChart/YAxis.js#L33 XAxis (incorrect): https://github.com/grommet/grommet/blob/master/src/js/components/DataChart/XAxis.js#L27 ### Actual Behavior XAxis tries to use data value as index....