Alternative implementation of a --no-github option

This PR adds support for a `--no-github` option to the `jeweler` command, which enables a user to create a gem project using Jeweler without requiring GitHub for a remote repository. When the `--no-github` flag is not set, the functionality of the `jeweler` command is unchanged; this PR does not introduce any breaking changes.

Unlike #186, this PR does not conflate 'no GitHub' and 'no Git.' Thus, using the `--no-github` option will still create a Git repo. Adding support for a `--no-git` flag could be added separately, as discussed in #186.

Usage:

```sh
jeweler --no-github the-perfect-gem
```

which will create a new gem project `the-perfect-gem` with an initialized Git repo, without a remote repository configured.

This PR also includes several tests for the new functionality.