Load the Jinja2 'do' expression-statement extension.
Added tests for jinja2 do extension and command line set.
Test (not just load) jinja2 do extension.
Merge pull request #762 from hjoliver/jinja2-do-extension

Load the Jinja2 'do' expression-statement extension.
Merge branch 'master' into release-prep-5.4
Change log updated.