Pass configuration via stdin

For integration into ci chains it would be great to be able to pass the configuration via stdin to the backstop command which would allow to create the configuration on the fly in non js environments.

I already tried to use /dev/stdin which theoretically could work via `someCustomScript | backstop test --config=/dev/stdin` but ran into the problems below.

```
BackstopJS v5.0.7
Loading config: /dev/stdin

/dev/fd/0:2
"id": "Vendor_Site",
^

SyntaxError: Unexpected token ':'
at wrapSafe (internal/modules/cjs/loader.js:1070:16)
at Module._compile (internal/modules/cjs/loader.js:1120:27)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1176:10)
at Module.load (internal/modules/cjs/loader.js:1000:32)
at Function.Module._load (internal/modules/cjs/loader.js:899:14)
at Module.require (internal/modules/cjs/loader.js:1042:19)
at require (internal/modules/cjs/helpers.js:77:18)
at loadProjectConfig (/Users/MFI/.nvm/versions/node/v12.16.2/lib/node_modules/backstopjs/core/util/makeConfig.js:46:20)
at makeConfig (/Users/MFI/.nvm/versions/node/v12.16.2/lib/node_modules/backstopjs/core/util/makeConfig.js:62:38)
at module.exports (/Users/MFI/.nvm/versions/node/v12.16.2/lib/node_modules/backstopjs/core/runner.js:5:16)
```

The same command works when used with temporary files `someCustomScript > backstop.json && backstop test --config=backstop.json` but it would be really good to avoid the temporary file.