feat: parse inline span tags and css color attributes in them

[mfontanini]

This PR adds support to parse inline span html tags and the color and background-color CSS attributes in the style span tag attribute.

The color and background-color can have any #RRGGBB formatted color + any color like red, dark_red, etc, as used in themes.

For now this only supports "strict" mode in which any deviation from what's supported will raise an error. In another PR I will add a way to configure this flag so if you don't feel like erroring, you can disable it.

After this change, the following:

# <span style="color: black; background-color: yellow"> _hi_ </span> mom

Paragraphs can <span style="color: #ffff00">**mix** colors <span style="background-color: white">in any way</span>


* <span style="color: red">this is red</span>
* this has <span style="background-color: blue">blue background</span>

Renders as:

Fixes #88