Optionally use Jinja2 templates for notification rendering

- Defaults are provided so that GitTail can be used without Jinja2.
- data dict is used to a greater extent to share template variables.
Add default Jinja2 notification templates

Template files which provides identical results to current
Jinja2-independent implementation.
For console output, indent consecutive lines based on first line's timestamp

Possibly somewhat more readable

Template-independent version is intentionally not given this feature
- Partially as a demonstation of customizability
- Partially as a hint that template-independent rendering exists
  only to provide a baseline of functionality, without ambitions
  to be feature-complete
Add config option to use a custom template path
Optionally determine custom template path from command line arguments