Allow ExpandIcon to be overridden

[hrgui]

Use case
Need to use a different icon for the expand button.

Ideas

Add a rendering method expandCellRender

1. Add new property expandCellRender, PropTypes.Function that has the following props: { prefixCls, expanded, record, needIndentSpaced }
2. If expandCellRender is passed, then instead of rendering ExpandIcon, just call expandCellRender. https://github.com/react-component/table/blob/master/src/ExpandableRow.js#L69

advantage: allows flexibility
disadvantage: requires work in rc-table

Option 2, requires no work on rc-table

• override rc-table-row-expand-icon rc-table-row-expanded and rc-table-row-expand-icon rc-table-row-collapsed via CSS

advantage: no work in rc-table
disadvantage: content is text only

[sytolk]

@hrgui do you have an example for Option: 2? How to override css classes used in react component

[hrgui]

Closing since its resolved in #236

[sytolk]

This fix not work with children