Feature request: Get background color of cell

I love camelot, it's been surprisingly reliable and it has been a big quality of life improvement for me 👏

I am dealing with a table where the text in the cell has a different meaning depending on the background color of the cell.

Feature request: Expose the background color of the cell.

Example how the camelot API could expose this:

```
tables = camelot.read_pdf("foo.pdf")
text_data = df = tables[0].df
background_color_data = tables[0].background_colors.df
```

Is this completely out of scope or would you accept a PR with this feature?

Start by tracing the existing camelot.read_pdf entry point and the table object exposed through tables[0].df. Determine where cell formatting is represented and how a background_colors.df property could fit the public API; done means callers can retrieve background colors aligned with the extracted cells.

Written by the indexing model from the issue text.