Add support for text, XML, HTML and XHTML Report
relatorio by default has support for those text format.
trytond could support them without too much modification:
- Add in ActionReport.template_extension
- Add in trytond.report.MIMETYPES
- Make Report.render return a str/bytes in such case (or maybe it should be relatorio)
- TranslationSet should be improved to parse translation of text (and to ease adding new report format)