Add libxml2 etc

Hiroaki Nakamurarequested to merge
hnakamur-guest/universal-ctags:add-libxml2-etc into master

This merge request makes ctags handle more xml-based languages. This is recommended by one of the upstream committer at https://github.com/universal-ctags/ctags/issues/655#issuecomment-377699060

Could you review this merge request? If you merge this, please merge !1 (closed) first.

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