[markdown mode] Support Unicode Punctuation Characters
[markdown mode] Support Unicode Punctuation Characters

Fix: `**LOREM(xxx)**：Fullwidth Colon` in [CodeMirror Markdown Demo](https://codemirror.net/mode/markdown/) results wrong style:
![](https://i.loli.net/2018/10/25/5bd1642fb2e01.png)

CommonMark Demo: https://spec.commonmark.org/dingus/?text=**LOREM(xxx)**%EF%BC%9AFullwidth%20Colon

See: https://spec.commonmark.org/0.28/#punctuation-character
wingify/CodeMirror,mtaran-google/CodeMirror,wingify/CodeMirror,thomasjm/CodeMirror,codio/CodeMirror,namin/CodeMirror,hackmdio/CodeMirror,notepadqq/CodeMirror,codio/CodeMirror,dperetti/CodeMirror,mtaran-google/CodeMirror,SidPresse/CodeMirror,wingify/CodeMirror,dperetti/CodeMirror,hackmdio/CodeMirror,jkaplon/CodeMirror,nightwing/CodeMirror,nightwing/CodeMirror,jkaplon/CodeMirror,namin/CodeMirror,mtaran-google/CodeMirror,dperetti/CodeMirror,notepadqq/CodeMirror,namin/CodeMirror,SidPresse/CodeMirror,thomasjm/CodeMirror,codio/CodeMirror,SidPresse/CodeMirror,hackmdio/CodeMirror,pabloferz/CodeMirror,jkaplon/CodeMirror,pabloferz/CodeMirror,pabloferz/CodeMirror,nightwing/CodeMirror,notepadqq/CodeMirror,thomasjm/CodeMirror