use WP HTML5 markup for captions instead of Soil

Use add_theme_support to enable native HTML5 captions and add caption styling to wp-caption-text.
Merge pull request #1105 from kevinlisota/html5-captions

use WP HTML5 markup for captions instead of Soil
Remove unnecessary string concatenation.
Check sidebar conditional tag exists

Return false for non-existent conditional tags. Fixes #993
Merge pull request #1113 from roots/conditional_checks

Check sidebar conditional tag exists