Provide built-in templates

Based on #43, I thought it would be a good idea to provide build-in templates. Currently, I can either use the [default app](https://github.com/darekkay/static-marks-app/) or provide a custom template via `--template-file` parameter. There is an [example template](https://github.com/darekkay/static-marks/blob/master/docs/examples/templates/custom.html) for others to apply or to use as inspiration to create custom templates. However, it would be nice to provide built-in templates so that it's not required to set anything up locally, e.g.:

```bash
static-marks build --template default [...]
static-marks build --template minimal [...]
```

Start with docs/examples/templates/custom.html and the existing --template-file behavior, then compare them with the default app linked in the issue. Trace the static-marks build command and determine how the default and minimal template names should be resolved. Done means both example commands work without requiring local template setup.

Written by the indexing model from the issue text.