Show build information in version output

[ferki]

When rex is installed from git either manually, or via OS vendors (like Gentoo's live ebuild), it would be super useful to make that fact clear by e.g. including extra information in the rex -v output.

For the extra information the following ideas emerged so far (not strictly in this form):

• git info, like < git:master@asdefg >
• build date

The proposed way to do that is to include variable(s) for that similarly to our $VERSION and replace the contents during build time. Then the version printing logic can check for the presence of the information and display it if needed.

[kentfredric]

Also worth mentioning: Some tools have different behaviour for -v and --version, -v is usually more terse and machine readable, while --version is permitted to have more verbosity.

If there's a way to do this, it might be nice.