Make overriding build-in XSLT possible

For different environments, a specific html rendering may be necessary or
desirable. If getXsltStream is private, an error-prone caller search and
replace is required. Therefore, it is preferrable to make the method
overridable directly.
Merge pull request #67 from Norwae/allow_xslt_replacement

Make overriding build-in XSLT possible