Add `markerComponent` prop to Bar

[haxxxton]

Describe the solution you'd like
Currently Bullet has a markerComponent prop that allows a custom renderer to be passed in for Markers. This prop doesnt exist on Bar. The proposal is to add the functionality to Bar to allow custom Markers to be rendered.

Describe alternatives you've considered
In my particular use case im trying to bold part of the text in the marker legend. An alternative would be to allow a custom legend renderer for CartesianMarkers (rather than just string for legend), but im unsure if a custom legend renderer is consistent with the @nivo API on other charts

Additional context
Below is an image of what im looking to achieve. Currently ive managed all but the change in font weight