Install git hook to automatically update asdf-standard submodule

[drdavella]

This PR adds some logic to setup.py to install git hooks for post-checkout and post-merge. The post-checkout hook makes sure that the asdf-standard submodule is updated automatically whenever a new branch is checked out. The post-merge hook should make sure that asdf-standard is updated automatically whenever pulling from a remote.

On the one hand, I think this is pretty clever (if I say so myself). On the other hand, it's maybe too clever, especially since git client hooks can introduce security vulnerabilities. (These particular hooks don't, but the possibility seems to make people wary).

cc @Cadair

[stsci-bot]

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog. I help make sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

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[coveralls]

Coverage remained the same at 95.106% when pulling 7ea4d74 on drdavella:submodule-update-hook into e8cd5a5 on spacetelescope:master.