Update hierarchy comps

[trainorpj]

This is related to #139

This PR does the following (each statement applies to both <Tree /> and <Cluster /> components:

1. Adds className to each <Group /> component.

• Is this where the className prop should go? I didn't see any other reasonable place to put it.

2. Adds <DefaultLink /> and <DefaultNode /> components to each component, along with basic tests for each.

• These render just fine--let me know if there are issues with them.
• I don't know much about testing components, so let me know if we need more comprehensive tests right now 🃏

This PR does not

3. Add defaults to the size, nodeSize, and separation props.

• The defaults are set in the corresponding d3 functions. The questions is, are we setting defaults at the highest or lowest level they're called at?
  • For example, there is no default for the top prop in either component--the prop gets passed to <Group /> which does have a default for top. So the default is set there (that's what I mean by "lowest level". I think this is the best way to do it.

4. Update the Readme.

• I can do this once the above points are addressed (let me know and I can add them before you merge 👍)

This is my first contribution to the codebase outside of documentation, so let me know if there's something I missed. For example, it doesn't have today's commits to master (I probably need to do that, right? 😬).

[hshoff]

let's change this to

className={cx('vx-cluster', className)}

that way it has a default class and folks can add their own

[trainorpj]

Got them both. I also found a (small) bug in my code, adding className where it has no business being 🐛

[hshoff]

same here

className={cx('vx-tree', className)}

[trainorpj]

I'll update the readme when this is merged (I need to have working links to the components).