Fix unintended syntax highlight overrides

I simply wanted to set default values for those highlight groups, in case it
was not defined already by the user ... But in fact it was overriding whatever
values were set previously.

plugin/rainbow_levels.vim

@@ -2,11 +2,11 @@ com! RainbowLevelsOn     call rainbow_levels#on()
 com! RainbowLevelsOff    call rainbow_levels#off()
 com! RainbowLevelsToggle call rainbow_levels#toggle()
 
-hi! link RainbowLevel0 Comment
-hi! link RainbowLevel1 Constant
-hi! link RainbowLevel2 Special
-hi! link RainbowLevel3 Identifier
-hi! link RainbowLevel4 Statement
-hi! link RainbowLevel5 PreProc
-hi! link RainbowLevel6 Normal
-hi! link RainbowLevel7 NonText
+hi default link RainbowLevel0 Comment
+hi default link RainbowLevel1 Constant
+hi default link RainbowLevel2 Special
+hi default link RainbowLevel3 Identifier
+hi default link RainbowLevel4 Statement
+hi default link RainbowLevel5 PreProc
+hi default link RainbowLevel6 Normal
+hi default link RainbowLevel7 NonText