Add colored ticks version

[apretto]

This PR adds a command line option "-c fire" to output the graph colored in fire (smallest tick is yellow, largest is red).

Adding additional palettes should be easy: just add a colon-separated string to the ticks_style array and a option statement for the case of the c parameter setting the correct index in the variable tick_style

[holman]

Do you mind dropping in a quick screenshot here?

I'm a little concerned about bloating things a little bit much (I don't think we'd need multiple color schemes, for example... maybe just a -c option), but I'm not totally 👎 on this yet either. Would be interested to hear what others think on this.

[apretto]

[MaxWorgan]

👍 came here to see if anyone had added this

[sindresorhus]

👍 Added this to my JavaScript port.

[abrist]

@apretto I was inspired by your commit and subsequently fattened up the color features in my PR: #94

[neuhaus]

can we make the color range configurable (perhaps as start and end values in HSV)? I'd like it to go from green to red for example.

[abrist]

@neuhaus See my PR and branch for the requested feature: #94

echo 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 | ./spark -p 52,88,160,166,172,178,148,82

[apretto]

I'm fine with closing this PR and going with @abrist's implementation (#94).

[abrist]

@apretto Thank you so much for your time, work, and inspiration!

[apretto]

Closing this PR finally.