Refactor: make the grapher usable as a library

[haidaraM]

I wrote most of the grapher almost three years ago in 2017 and lot of things happened since 😄

Currently, the Grapher is tightly coupled to the CLI. This makes it difficult to use it as a library inside another project. Also the Grapher is a little bit complicated: thinking of splitting it. I also want to make the graph customizable by not hardcoding the shapes, layout and make the post processor more extensible.

So this issue is here to track the related changes. The outcome of these changes will be the version v1.0.0 of the grapher?

[haidaraM]

The code is now cleaner. Will update the Readme with an example to integrate this in an existing project. Meanwhile, you can take how the CLI works as an example.

[haidaraM]

For those following this issue: while the code can be used in another project, keep in mind that the internal API is not stable yet. I will likely refactor the internal of the grapher in the following versions. The Parser should not change that much.