fix critical bug in metaeditor

    files in bare .html format (with metadata but not parsed or disambiguated)
    were damaged by the meta.py on open (text content deleted).

    Bug was in FileWrapper class (formats.py) which ignored
    "dummy .html" variety of format treating it as a parsed one.

    Fix: new .isdummy attribute is added for HtmlReader and
    TxtReader classes, set in True for TXT always and for Html
    when the "dummy .html" file is detected.