Add support for customising a card title color

[MitchellCash]

Currently the card title will use the card body colour for its text. This pull requests lets you set a different card title colour compared to the body, which I find useful especially when using the new CSS variable --#{$prefix}card-title-color.

Personally I prefer this approach versus adding a utility class like text-primary as it allows for greater customisation via the SASS/CSS variables.

If this is useful I can also add a commit for the card subtitle?

[julien-deramond]

Sorry for the delay @MitchellCash. We were working on the v5.2* until now :)

On my side, I find this request legitimate since we already have --bs-card-color and --bs-card-cap-color so let's go for more customization. In the same spirit, adding the same thing for card subtitle color might be useful as well.

@mdo Thoughts about this change? Any strong opinions against it?

[MitchellCash]

@julien-deramond before merge I will add the card subtitle colour as well (working on that now)

[MitchellCash]

Ready for review/merge now 😃 Please let me know if you need me to squash any commits etc.

[julien-deramond]

Ready for review/merge now 😃 Please let me know if you need me to squash any commits etc.

Thanks for that @MitchellCash. Just reviewed it, it is still OK for me. Don't worry about squashing the commits, we can do it while merging with the GitHub interface.
I let @mdo double-check if it is compliant with what we are trying to achieve with CSS vars in the future.