Added prepareForReuse functions to Button and Label bricks

[willspurgeon]

Issue #187

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@fa2d3ad). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #188   +/-   ##
=========================================
  Coverage          ?   93.48%           
=========================================
  Files             ?       40           
  Lines             ?     4438           
  Branches          ?      361           
=========================================
  Hits              ?     4149           
  Misses            ?      288           
  Partials          ?        1

Impacted Files	Coverage Δ
Source/Bricks/Label/LabelBrick.swift	85.18% <100%> (ø)
Source/Bricks/Button/ButtonBrick.swift	86.36% <100%> (ø)

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