[React] Add fill prop

[colebemis]

Problem

Our octicon components don't provide a way to change the fill attribute of the underlying SVG.

Solution

This PR adds a fill prop to all octicon components. @macno originally implemented this in #418 but merge conflicts made that PR difficult to merge.

Co-authored-by: @macno

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[jonrohan]

Mostly want to know what others think about adding fill. Does this mean someone will be able to color the icon with fill and with color props?

[colebemis]

@jonrohan Our components don't currently accept a color prop. Right now, the only ways to change the color of an icon with the React library is to style an icon with CSS or change the color of a parent element.