Add Zika example to vignette

[adamkucharski]

• Please check if the PR fulfills these requirements

• [X ] I have read the CONTRIBUTING guidelines
• [ X] A new item has been added to NEWS.md
• [ X] Tests for the changes have been added (for bug fixes / features)
• [ X] Docs have been added / updated (for bug fixes / features)
• [X ] Checks have been run locally and pass

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Update to documentation.

• What is the current and new behavior? (You can also link to an open issue here)

The current vignette is focused on dengue only. This PR adds a French Polynesia Zika case study to the climateR0.Rmd vignette, as well as an accompanying dataset. This PR also unhides plotting for dengue to make the vignette easier to follow on a web page.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.