Support upgrading via a manually downloaded binary

[eschrock]

Problem

Not everyone may have permission within their corporate setting to have titan automatically download a binary from github.

Solution

Provide a way for them to download the updated titan binary via some other approved means, and the run titan upgrade to switch over to it. This could just be a matter of
documenting the --finalize behavior, or we may want to provide a --binary path
to point to a newer version on their system.

Alternatives

Switch by hand, then titan uninstall / titan install