Markdown text color

[EmmanuelDodoo]

This PR adds an option for setting the text color of rendered markdown. markdown::Style has been updated to include two new fields, text_color to set the color for basic text and strong_color for strong and emphasized text color

[EmmanuelDodoo]

Ideally, there would be some granularity in assigning text colors so a single rendered markdown could have paragraphs with different colors.

I attempted this approach by having markdown::Text pass its color to rich_text but that meant requiring the additional bound from rich_text::color_maybe. My proposed solution is to add an additional text_color method to text::Catalog to remove the need for extra bounds though I wanted to discuss the idea before moving forward.