New forest plot label functionality

[thomrawson]

Pull Request

A demo PR that adds the ability to define custom y-axis labels - for instances where it is visually preferable to remove separate item indicators from the article_label.

Checklist:

• [ NA] I have added new dependencies in DESCRIPTION
• [ YES] I have bumped the version number in DESCRIPTION
• [ YES] I have updated the NEWS.md
• [ ?] I have added unit tests using testthat
• [ YES] I have checked that the package can install locally using devtools::install_local("<path-to-epireview>")
• [ YES] I have added documentation where required
• [ NA] My documentation includes examples of how to use the function
• [ NA] If necessary, I have updated priority-pathogens

[thomrawson]

First few commits add custom label functionality. Following commits then replace that with more effective functionality that Sangeeta helped with. The rest are quite small and self-explanatory.