export RelayCompilerMain (#2800)
export RelayCompilerMain (#2800)

Summary:
This exports the function exposed by `RelayCompilerMain` to allow consumers to run relay compiler through a node api.

Our use case for this is a custom toolkit package (similar to react-scripts) the encapsulates our logic for running relay-compiler (and other scripts) with the correct settings for our environment. Since our toolkit knows all the correct options to be passed to relay compiler, we'd like to bypass the `relay.config.js` file and instead pass in the options directly as a javascript object.

```js
const { relayCompiler } = require('relay-compiler');

relayCompiler({
    schema,
    src,
    extensions: ['js', 'jsx'],
    // ... etc
}).catch(e => {
    console.error(e);
});
```
Pull Request resolved: https://github.com/facebook/relay/pull/2800

Test Plan:
jestflow

also see PR

Reviewed By: jstejada

Differential Revision: D16344889

Pulled By: josephsavona

fbshipit-source-id: 0a5d7e61829afccbdbd64d07df7fc43e6cfd3a31

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