added option to choose default/darker shimmer colors

[ema987]

fixes issue #20

[elye]

Good change.
I suggest don't need to have 2 custom attributes (defaultColor, darkerColor), but instead of one (customColor).
And when that exist, both bold and non bold would use that instead of the default one.
If possible, make one of the text line in the sample app to use the customColor, and amend the readme to have the feature.
We want to minimize the custom attributes to make it easier to use.
Thanks.

[ema987]

Hi @elye, thank you for your reply!
I've made changes following your suggestions. Code is basically as previous commits besides that now custom_color attribute will override both defaultColor and darkerColor used in the CustomViews.
I've updated the readme, too.
It should be ok now, but if it's not, just let me know!