Add support for custom report colours

[practical-data-science]

[practical-data-science]

When creating a new report you can now define the background colour for chapters and tiles, and set the font colour:

# Create a report
pdf = report.Report(output='example.pdf')

pdf.set_title('Gilfoyle example')
pdf.set_accent_background_color('#9f85ca')
pdf.set_accent_font_color('#fff')

# Create an empty payload
payload = pdf.get_payload()