fix: add missing markup highlights

Uses new strikethrough variable, see b5e21ff

Fixes: #117

after/plugin/srcery.vim

@@ -43,6 +43,7 @@ let s:italic = g:srcery#palette.italic
 let s:underline = g:srcery#palette.underline
 let s:undercurl = g:srcery#palette.undercurl
 let s:inverse = g:srcery#palette.inverse
+let s:strikethrough = g:srcery#palette.strikethrough
 
 " }}}
 " Sneak: {{{
@@ -284,6 +285,11 @@ if has('nvim')
       highlight! link @text.uri TSURI
       highlight! link @variable TSVariable
 
+      call srcery#helper#Highlight('@markup.strong', s:none, s:none, s:bold)
+      call srcery#helper#Highlight('@markup.italic', s:none, s:none, s:italic)
+      call srcery#helper#Highlight('@markup.underline', s:none, s:none, s:underline)
+      call srcery#helper#Highlight('@markup.strikethrough', s:none, s:none, s:strikethrough)
+
     endif
   endif