Add dataset versioning

[mitchelloharawild]

Allow users to install a specific version of a dataset, if the obtainer function supports it.

[FinYang]

Package writer: someone who uses roam to write packages. User: someone who uses the package Package writer wrote (not roam).

• [x] Restrict the first argument of the obtainer function to be the version number user wants to install
• [x] In the roam_install function, allow user to specify which version they want to install, which we pass to the obtainer function. If unspecified, we pass NULL so the obtainer knows to install the latest version
• [x] Provide a roam_version function to package writer, which they can use in the obtainer function to specify the currently downloaded version number, which we save to a text file in the cache folder
• [x] roam_update is a wrapper around roam_install which passes NULL as the version number to install the latest version.