Added shellcheck_aspect

[UebelAndre]

This change adds the shellcheck_aspect rule for running shellcheck within an aspect.

To configure this the following should be added to a repo's .bazelrc

build --aspects=@rules_shellcheck//shellcheck:shellcheck_aspect.bzl%shellcheck_aspect
build --output_groups=+shellcheck_checks

Then any bazel build invocation will spawn Shellcheck actions for sh_binary, sh_test, and sh_library targets.

closes #23