Support application version via env variable

[miraage]

I assume this is a pretty common case to display application version somewhere. For reproducing the bugs, QA purposes and so on. If you ask me there is no concise way to put application version to the bundle, except duplicating it somewhere.
I think REACT_APP_VERSION variable name suits the needs. On the other hand, this variable will be defined implicitly, which is not very clear for the developer unless he always reads notes/docs.
I'm ready to provide a PR if you agree.

[Timer]

This will be covered by #2468; thanks though!