Add usage of license-file path for EULA if it has the RTF extension

[volks73]

If the license-file field is used to define a path to a custom license instead of the license field (See #57), then the file extension should be checked and if the extension is RTF, then the file should be used for the EULA in the license agreement dialog of the installer.

This allows a developer to include a RTF license and define it only in the Cargo.toml file to be used as the EULA and sidecar file without having to use the -E,--eula option (See #57) during initialization.

[volks73]

This has been added as of a3a4eed.