Add outbound links and file downloads tracking for plausible.io (#774)

### Prerequisites

Put an `x` into the box(es) that apply:

- [ ] This pull request fixes a bug.
- [x] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

Configuration options to activate [outbound link click
tracking](https://plausible.io/docs/outbound-link-click-tracking) and
[file downloads
tracking](https://plausible.io/docs/file-downloads-tracking).

These are two standard options for Plausible that are convenient and
useful in many circumstances.

### Issues Resolved

List any existing issues this pull request resolves.

### Checklist

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#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [ ] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [ ] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already

---------

Co-authored-by: Luiz F. A. de Prá <luizdepra@users.noreply.github.com>