Support emoji fonts

Hi, I recently studied some emoji fonts. These emoji fonts have 
fontconfig difficulties when packaging. They try to set the font as 
default emoji font but do not affect others. Usually the font has a 
separated configure file.

<match>
   <test name="family">
     <string>sans-serif</string>
   </test>
   <edit binding="strong" name="family">
     <string>Nimbus Sans L</string>
     <string>EmojiOne Color</string>
   </edit>
</match>

However, this will affect sans-serif font settings of other font 
packages or users' setting, because the package do not know which 
sans-serif font users want to use.

I suggest maybe we can map the Unicode emoji block as test condition. 
Just like when we set a Japanese font, it won't affect English and 
Arabic fonts.

<match>
   <test name="family">
     <string>sans-serif</string>
   </test>
   <test name="lang">
     <string>emoji</string>
   </test>
   <edit binding="strong" name="family">
     <string>EmojiOne Color</string>
   </edit>
</match>



-- 
Guo Yunhe
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