The default downloadTool implementation strips the filename so when passing it to the extractTar function, it doesn't have a way of knowing the format without manually specifying it. However what the extractTar function arguments meant and how to specify them wasn't clear before reading the source code, so the documentation here got updated to reflect that.
* added test for extracting .tar.gz
* added ability to extract .tar.xz
* add flags to extract tar
* make use of tempPath
* check file contents and make different content for tar file
