Make link end marker's width and height configurable

[danielcaldas]

Is your feature request related to a problem? Please describe.
Currently there is no way for users to specify the width and the height of link's [marker[(https://developer.mozilla.org/en-US/docs/Web/SVG/Element/marker)'s elements. As you can see in our codebase, this values are hardcoded, see for yourself in src/components/marker/Marker.jsx.

<marker
    className="marker"
    id={this.props.id}
    viewBox="0 -5 10 10"
    refX={this.props.refX}
    refY="0"
    markerWidth="6"           // width
    markerHeight="6"          // height
    orient="auto"
    fill={this.props.fill}
>

Describe the solution you'd like
The propriate solution here would be for the library to expose this as a link-level configurable value.

So basically, extend the existent functionalities in src/components/graph/graph.config.js adding two new properties:

// ...
link: {
    // ...
    markerWidth: 6,
    markerHeight: 6,
    // ...
}
// ...

Additional context
Changing marker's width and height, impact.

[SachinVarghese]

@danielcaldas I would like to work on this. Can you explain how are these markers being rendered from SVG defs (using markerIds right)?

Edit: Alright I read through the codebase to figure stuff out. I will submit a PR soon. Please review

[danielcaldas]

Done in #245