`inverse_header_color` should inherit `inverse_text_color`

[Garrick Aden-Buie]

And there might be other places where choose_dark_or_light() was called multiple times in a family of arguments where it could only be called once.

xaringanthemer/data-raw/theme_template_variables.R

Lines 19 to 21 in 3a1d9f3

 , "inverse_text_color", "#d6d6d6", ".inverse", "Inverse Text Color", "--inverse-text-color" 
 , "inverse_text_shadow", "{FALSE}", ".inverse", "Enables Shadow on text of inverse slides", NA_character_ 
 , "inverse_header_color", "#f3f3f3", ".inverse h1, .inverse h2, .inverse h3", "Inverse Header Color", "--inverse-header-color"