Add testing

[wyozi]

There's currently a very limited set of tests for e2e pptx generation. Since these tests are a relatively nice way of ensuring that library functionality does what's expected, it would be nice to have many more of them. See https://github.com/wyozi/react-pptx/blob/master/src/index.spec.tsx#L205 for an example

Some test ideas:

• Test that every node type appears in the slide (Text, Shape etc)
• Test master slides
• Test percentage dimensions mapping to absolute values
• Test pptx layouts
• Test color mapping to .pptx colors

[muhammadbasitobaid]

Hi @wyozi , first of all, thanks for this library. I would be really honored to contribute to this issue, if that's fine with you. Also, I couldn't help but notice that the README.md could be improved as well.