texinfo output: @ref node name should not contain `.'

[joostkremers]

The auto_identifiers extension strips out most special characters from headings, but it explicitly leaves periods in. Texinfo does not like periods in @ref node names, however, so preferably the texinfo writer should replace periods with something else.

Originally posted on the mailing list: https://groups.google.com/forum/#!topic/pandoc-discuss/A7_jXnqqEow