Fixes positioning issue #118 (add horizontalAdjustment prop)

[topherPedersen]

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This pull requests adds a new prop, horizontalAdjustment, similar to the existing topAdjustment prop, which allows horizontal adjustment of the highlighted child view should the library get the alignment wrong. I ran into the same issue described in Issue #118 yesterday, a horizontal alignment problem, and created this new prop to solve this issue in my organization's app.