Add "theme_option" helper method

[kaidesu]

After renaming theme settings to theme options, we should add a supporting theme_option helper method to fetch the values of theme options for use within themes.

This is currently handled by the theme helper method, but you have to reference options with a prepended value key:

{{ theme('value.homepage.message') }}

This is just not developer friendly and unintuitive vs.

{{ theme_option('homepage.message') }}