Fixes #1813 [ci skip]

This ensures that the final repository URL cloned will have the
repository's actual name after redirect.

In `hub clone NAME`, "NAME" was previously considered a local resource
if a directory of the same name existed, and the clone command was left
unchanged.

That worked for a while, but some users were surprised that they
couldn't clone their repo named "NAME", only to discover that there was
an unrelated directory "NAME" in the current working directory.

Also, a git bundle is a valid cloneable resource, but is a file and not
a directory.

This refines the detection of cloneable resources. "NAME" is considered
to be locally cloneable if one of the following is true:

- "NAME/.git" exists
- "NAME" is a bare git repo
- "NAME" is a file whose first line includes "git bundle"

If it doesn't exist, print a more helpful error message about it so the
user knows which "owner/repo" pair was tested.

`configService` can save and load `Config` using a strategy of `configEncoder` and `configDecoder`. This pave the path for swapping out `tomlConfigEncoder` and `tomlConfigDecoder` with `yamlConfigEncoder` and `yamlConfigDecoder`.

`Configs` also has been renamed to `Config`.

This'll make sure we propagate error to the caller where we can reformat
the error message within the context.

This fixes `features/authentication.feature:85`

See comment https://github.com/github/hub/pull/515#discussion_r10915319

* Rename Credentials to Credential
* Use toml for config

- Use a hash to lookup for commands.
- Add more tests for the command and runner execution.