Add Eslint, Add Prettier, Add Flow and a .editorconfig

[dawidvdh]

These tools are there for code stability, cohesion and are especially helpful when more then a single person works on a project.

• Eslint not only improves code quality but ensures that the code is cohesive when multiple parties work on it.
• Prettier ensures that all the code is always formatted according to project standards.
• Flow is a static type checker which will make the code type safe eliminating bugs and making the code self documenting to a degree.
• .editorconfig just ensures that anyone working on the app will use spaces instead of tabs etc