Add image syntax for text color

[yhatt]

This PR adds image syntax for text color ![](color) to provide consistent interfaces for setting color via shorthand by using image syntax.

# Hex color (White BG + Black text)

![bg](#fff)
![](#000)

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# Named color (rebeccapurple BG + White text)

![bg](rebeccapurple)
![](white)

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# RGB values (Orange BG + White text)

![bg](rgb(255,128,0))
![](rgb(255,255,255))

We've updated documentation of image syntax too. Resolves #154.