Support for offline mode with previously installed bazelisk

Currently if I run bazelisk, when my PC is in offline mode, it fails to run my build/test tasks as it is not able to connect to the internet and validate that it is running the latest version.

$ ./tools/bin/bazelisk-darwin-amd64 help
2019/09/08 08:15:36 could not resolve the version 'latest' to an actual version number: could not get releases from github.com/bazelbuild/bazel: could not download list of Bazel releases from github.com/bazelbuild: could not fetch https://api.github.com/repos/bazelbuild/bazel/releases: Get https://api.github.com/repos/bazelbuild/bazel/releases: dial tcp: lookup api.github.com: no such host
$

It would be great if it could simply print a warning message about this and continue with running the bazel build/test tasks. And perhaps provide an option to run bazelisk in offline mode so that it doesn't have to print warning message.

Start by tracing Bazelisk's version-resolution path and its GitHub release lookup, then reproduce the reported ./tools/bin/bazelisk-darwin-amd64 help failure without network access. Done means a previously installed version can run build/test tasks offline, with a warning or an offline option as appropriate; add coverage for the offline case if the existing test structure supports it.

Written by the indexing model from the issue text.