Implement AuthorizedPrincipals command

Closes #181

See merge request gitlab-org/gitlab-shell!322

Build this command when `Executable` name is
`gitlab-shell-authorized-principals-check`. Feature flag is the
same name.

Implement AuthorizedKeys command

See merge request gitlab-org/gitlab-shell!321

Build this command when `Executable` name is
`gitlab-shell-authorized-keys-check`. Feature flag is the same
name.

Support falling back to ruby version of checkers

See merge request gitlab-org/gitlab-shell!318

This struct is responsible for determining the name and
root dir of the executable.

The `RootDir` property will be used to find the config.

The `Name` property will be used to determine what `Command`
and `CommandArgs` to be built.

Other functions are still expecting for `CommandArgs` instead
of `Shell`. They should be expecting `commandargs.Shell` now
since it has been renamed.

`CommandArgs` has been renamed to `Shell`.

An interface has been added that includes `Executable()` and
`Arguments()` method. The `BaseArgs` implement this methods
and should be embeeded in each type.

Rename the ruby scripts to have `-ruby` suffix and add a symlink
for both to `./gitlab-shell`. The executable name will be used to
determine how args will be parsed.

For now, we only parse the arguments for gitlab-shell commands. If
the executable is `gitlab-shell-authorized-keys-check` or
`gitlab-shell-authorized-principals-check`, it'll always fallback
to the ruby version.

Ruby specs test the ruby script, the fallback from go to ruby and
go implementation of both (still pending).

Update rubygems version on CI for go tests

See merge request gitlab-org/gitlab-shell!320

Resolve "Update .PHONY to have accurate list of targets"

Closes #198

See merge request gitlab-org/gitlab-shell!316

Add Makefile for easier building and testing

Closes #194

See merge request gitlab-org/gitlab-shell!310

Set Go111MODULE to 'off' during compilation

See merge request gitlab-org/gitlab-shell!315

Given more to the Go community moves to Go modules, one might default to
GO111MODULE to on in their environment. If this is done, this project
fails to compile.

By setting the environment for the compilation this is fixed.

Enable all migration features by default

See merge request gitlab-org/gitlab-shell!313

In order to uncomment it in the Makefile of GDK

Release v9.3.0

See merge request gitlab-org/gitlab-shell!312

Go implementation for LFS authenticate

Closes #163

See merge request gitlab-org/gitlab-shell!308

Go implementation for git-upload-archive

Closes #162

See merge request gitlab-org/gitlab-shell!307

Respect GITLAB_SHELL_DIR in the Go version

Closes #187

See merge request gitlab-org/gitlab-shell!309

Go implementation for git-upload-pack

Closes #160

See merge request gitlab-org/gitlab-shell!305

Return Fallback cmd if feature is enabled, but unimplemented

See merge request gitlab-org/gitlab-shell!306

Go implementation for git-receive-pack

Closes #161

See merge request gitlab-org/gitlab-shell!300

Only build against go1.11 and go1.12

See merge request gitlab-org/gitlab-shell!304

Refactor execution and parsing logic in Go's implementation

See merge request gitlab-org/gitlab-shell!302