Automatically release tidylab package versions

[harell]

Each tidylab package has three branches: (1) master, (2) release and (3) develop.

• Developers push daily changes into "develop".
• Developers may push daily changes into "release".
• Developers may not push anything into "master".

Once a week, a "scheduler" will merge changes between "release" and "master", by:

1. changing DESCRIPTION, including package version.
2. changing README, including package version.
3. rendering README (including the future manual and vignette links. (see figure)
4. installing the package (use build_vignettes = TRUE when running install_github);
5. copying the rendered files (manual and vignettes) into "~/vignette".
6. rendering README
7. committing the new (weekly) version into "master" (save a copy of manual and vignette for 1 week)