Added flag to checkout to update submodules when present.

[patcon]

Addresses #13

Want me to add an entry to CHANGELOG as well?

[patcon]

I needed to add a "prepare" script in order to have npm install directly from git, but it now works! This helps when ppl want to submit PRs like this, and rely on it until you have time to review. It also allows them to more easily and visible assure you that it works in their CI scripts :)

Sorry, ideally that would be another PR, but I thought these might be confusing in isolation.

Context: https://blog.jim-nielsen.com/2018/installing-and-building-an-npm-package-from-github/

To see what the prepare script does, run npm install in any project at each commit in this PR, and see how diff the folder looks in node_modules

# WITHOUT prepare script
$ npm install patcon/storybook-branch-switcher#9985ca6 --save-dev
$ ./node_modules/.bin/sb-branch-switcher
zsh: no such file or directory: ./node_modules/.bin/sb-branch-switcher
$ tree node_modules/storybook-branch-switcher
./node_modules/storybook-branch-switcher
├── LICENSE
├── README.md
├── manager.js
├── package.json
└── preview.js

# WITH prepare script
npm install patcon/storybook-branch-switcher#614c032 --save-dev
$ ./node_modules/.bin/sb-branch-switcher
😎 Your workspace looks good! ...
$ tree node_modules/storybook-branch-switcher
./node_modules/storybook-branch-switcher
├── LICENSE
├── README.md
├── dist
│   ├── cli.mjs
│   ├── cli.mjs.map
│   ├── index.d.mts
│   ├── index.d.ts
│   ├── index.js
│   ├── index.js.map
│   ├── index.mjs
│   ├── index.mjs.map
│   ├── manager.mjs
│   ├── manager.mjs.map
│   ├── preview.d.mts
│   ├── preview.d.ts
│   ├── preview.js
│   ├── preview.js.map
│   ├── preview.mjs
│   └── preview.mjs.map
├── manager.js
├── package.json
└── preview.js

2 directories, 21 files

[patcon]

Hm. I'm seeing some odd behavior. The git submodule init step might also be needed, for when a new submodule appears in .gitmodules between branches

EDIT: The current version works better, and uses commands closer to git's direct commands, which is probably good.