hugolib: Only do link transforms etc. on HTML type of pages
hugolib: Only do link transforms etc. on HTML type of pages

See #2828

mdhender/hugo,fedelibre/hugo,anthonyfok/hugo,mitchchn/hugo,gohugoio/hugo,digitalcraftsman/hugo,mdhender/hugo,fedelibre/hugo,mitchchn/hugo,fedelibre/hugo,gohugoio/hugo,gohugoio/hugo,anthonyfok/hugo,anthonyfok/hugo,mdhender/hugo,digitalcraftsman/hugo,fedelibre/hugo,mdhender/hugo