Only import icons that are used

[rileyhilliard]

Describe the bug
This is part II of issue #106
react-digraph is importing all of react-icons, but it is only using one icon from font awesome.

To Reproduce
Steps to reproduce the behavior:

1. Checkout (or merge) Adding webpack-build-analyzer to react-digraph #107
2. yarn && yarn analyze-bundle
3. look at the dependency output

Expected behavior
react-digraph should only need to output the one icon that is being used in the final production build. Optimizing this is fairly straight forward, and will reduce the prod build of react-digraph by about 60%.

Screenshots

Desktop (please complete the following information):

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Additional context
I plan on fixing this myself. This issue is only for tracking purposes.

[rileyhilliard]

@ajbogh / @chiwoojo
I was able to optimize the production build footprint by 68% by only importing the expand icon that is being used by react-digraph. This reduces the GZipped size from 548kb to 174kb.
Once #107 is merged, I'll submit the pull request for the optimization above.