Closes#4284.
Headers with the corresponding tags should not appear in the output.
If one or more of the specified tags contains a non-tag character
like `+`, Org-mode will not treat that as a valid tag, but will
nonetheless continue scanning for valid tags. That behavior is not
replicated in this patch; entering `cat+dog` as one of the entries in
`#+EXCLUDE_TAGS` and running the file through Pandoc will cause the
parser to fail and result in the only excluded tag being the default, `noexport`.
