Add jinja templating support to utils.task_wrappers.run and extractor.metric_runner.MetricRunner

[rsyi]

For more modular SQL-writes within the platform, we should enable jinja templating in metrics queries (in MetricRunner) & raw queries (in run), using templates stored in the ~/.whale/templates folder.

Detailed specifications

• All Jinja templates should be stored in ~/.whale/templates.
• A template ~/.whale/templates/my_template should be able to be added to a query by inserting {% extends my_template.sql %}, following the Jinja template engine pattern (similar to the pattern found in Django). This will require invocation of a jinja2.FileSystemLoader with the path ~/.whale/templates given as argument (an example is shown here).
• Both utils.task_wrappers.run and extractor.metric_runner.MetricRunner.extract() should run the templating engine first, then execute the rendered query.

[rsyi]

Closed with #96