Easy legend adjustments

[jonocarroll]

Legend adjustments and helpers:

• easy_change_legend(what = c("position", "direction", "justification"))
• easy_move_legend(to = c("right", "none", "left", "bottom", "top"))
• easy_legend_at(c("right", "none", "left", "bottom", "top"))
• easy_rotate_legend(to = c("vertical", "horizontal"))
• easy_adjust_legend(to = c("left", "right", "center"))

Legend title builder:

• easy_add_legend_title()

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• [x] teach options
• [x] tests with 100% coverage
• [x] closes #26
• [x] closes #24

Any other suggestions @krlmlr?

[codecov[bot]]

Codecov Report

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  Lines             ?    135           
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  Hits              ?    135           
  Misses            ?      0           
  Partials          ?      0

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