Closes gruntwork-io#115

These changes add support for git-xargs registered environment variables
based on arguments and flags intended for use by the commands and
scripts called by git-xargs.  These environment variables start with
`XARGS_` to avoid collision with other use cases and include:

- `XARGS_REPO_NAME`: the name of the repository targeted
- `XARGS_DRY_RUN`: whether `--dry-run` flag was provided

Additionally, these changes contain a basic test to ensure this feature
works as expected going forward.

While updating documentation, I realized that the original intent of
issue 115 might require the repository owner information as well as the
repository name.  This expands the initial changes and performs some
minor refactoring.