src/rss_converter_dilbert.com.xsl: fix generating enclosures Enclosures were not generated for Dilbert.com because the URL of the picture are protocol-relative and curl cannot work with these URLs. Fix the URLs by prepending a protocol schema to them.
Add option to enable or disable showing multimedia content in RSS items Tweeper by default shows multimedia contents like Twitter and Instagram images in items descriptions. However sometimes just having multimedia contents in the <enclosure/> element may be enough, so make it optional to also have the content in the item description. Keep the current default behavior for backwards compatibility.
src/rss_converter_*.xsl: add missing generate-enclosure parameter XSL parameters do not necessarily need to be declared in the stylesheet if no default value is explicitly set, however tweeper is doing that for other stylesheet, so declare the parameter in rss_converter_pump.io.xsl and rss_converter_dilbert.com.xsl as well for consistency.
Remove unneeded attribute extension-element-prefixes from xsl stylesheets It looks like the "extension-element-prefixes" attribute is not strictly needed for php extension functions to work, so remove it. If it turns out that the attribute is actually needed in some cases it can always be added back.
Update copyright years
rss_converters_*.xsl: prefix the namespace when calling Tweeper class methods The Tweeper class is now in a namespace, without this change the XSLT processor would give errors like this: PHP Warning: XSLTProcessor::transformToXml(): Unable to call handler Tweeper::epochToRssDate() in .../src/Tweeper.php on line 356
tweeper: move the main Tweeper class to its own file under src/ This matches more closely the project structure expected by composer packages.