It will be nice to support pnpm

I have tried to install dependences with pnpm (https://pnpm.js.org/) but had the following error at the post install.

```
.../gifsicle/3.0.4/node_modules/gifsicle postinstall$ node lib/install.js
│ internal/modules/cjs/loader.js:638
│     throw err;
│     ^
│ Error: Cannot find module 'got'
```

No problem at all with yarn.