Add Markdown italic and bold delimiter highlighting

The groups "markdownBoldDelimiter" and "markdownItalicDelimiter" for the
delimiter of italic and bold formatted text are now highlighted as
keywords with "nord9". This also ensures that both styles are easily
recognizable even when the "nord_italic" (1) theme configuration is not
enabled.

References:

  (1) #89

GH-90

colors/nord.vim

@@ -361,13 +361,15 @@ call s:hi("markdownIdDeclaration", s:nord7_gui, "", s:nord7_term, "", "", "")
 call s:hi("markdownH1", s:nord8_gui, "", s:nord8_term, "", "", "")
 call s:hi("markdownLinkText", s:nord8_gui, "", s:nord8_term, "", "", "")
 call s:hi("markdownUrl", s:nord4_gui, "", "NONE", "", "NONE", "")
+hi! link markdownBoldDelimiter Keyword
 hi! link markdownFootnoteDefinition markdownFootnote
 hi! link markdownH2 markdownH1
 hi! link markdownH3 markdownH1
 hi! link markdownH4 markdownH1
 hi! link markdownH5 markdownH1
 hi! link markdownH6 markdownH1
 hi! link markdownIdDelimiter Keyword
+hi! link markdownItalicDelimiter Keyword
 hi! link markdownLinkDelimiter Keyword
 hi! link markdownLinkTextDelimiter Keyword
 hi! link markdownListMarker Keyword