Allow custom sidebars for Header Right widget area

Added functions to override `genesis_go_header` and display custom sidebar in Header Right widget area, defaulting back to Header Right (if active) if no widgets are present in custom sidebar.

I didn't re-create any .pot files. I only added one translatable string in `includes/term.php` line 71, and it can be removed and changed to `<?php echo esc_attr( $wp_registered_sidebars['header-right']['name'] ); ?>` if appropriate. I wasn't sure how you typically generate them (via the WordPress.org plugin repo admin page, the WordPress i18n tools, etc), but I can re-generate it if needed.