feat(outline): Make outline a separate prop

[TheSharpieOne]

The outline for buttons and cards is now it's own props separate from the color value.
Based on #33 (comment)

This also adds additional tests around this new props.

Also:
Cards, unlike buttons, do not have a default color. Thus adding outline prop to a card without a color will do nothing.
Is this acceptable? Should it produce a warning/error? Should it default to 'card-outline-secondary' when outline is provided but no color is provided?

[coveralls]

Coverage remained the same at 100.0% when pulling ee5fa1e on TheSharpieOne:feature/outline-prop into f617dc5 on reactstrap:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 7496c91 on TheSharpieOne:feature/outline-prop into b62ee01 on reactstrap:master.

[eddywashere]

@TheSharpieOne thanks a ton for the prs!

I think this is acceptable behavior. I think card already has a solid style without background variants applied where as the default .btn kind of looks weird and isn't really shown in the docs.

re: warnings/errors. I'd love to throw warnings in dev mode that catch issues like this and not include those warnings in production builds.