Persist "![" when an image opener was not matched.
Persist "![" when an image opener was not matched.

Fixes http://spec.commonmark.org/0.12/#example-452

AMDL/CommonMark.NET,lostfictions/CommonMark.NET,dudb/CommonMark.NET,laedit/CommonMark.NET,Mosoc/CommonMark.NET,laedit/CommonMark.NET,dut3062796s/CommonMark.NET,Mosoc/CommonMark.NET,AMDL/CommonMark.NET,dut3062796s/CommonMark.NET,lostfictions/CommonMark.NET,dudb/CommonMark.NET