Add global option `--offline` to prevent all network access

If the flag --offline is passed, rv should try to operate without making network requests. For example, it should not check for available ruby versions, try to install ruby versions, or try to look up or install gems. If it's possible to run without network requests (perhaps a ruby version that meets the requested version is already installed?) it should run with whatever is already present. If it's not possible to run without making network requests, rv should fail with an error explaining the problem and a suggestion to try again without --offline because that might work.