How to add custom IRTransforms and Parsers

`relay-compiler` README states that there is a possibility to write our own compiler (do things like swapping or adding a `FileParser` and additional `IRTransforms`)
It says:

> To actually run your compiler, you will also need a script to assemble all the above components. A sample file can be found [HERE](https://github.com/facebook/relay/blob/master/packages/relay-compiler/bin/RelayCompilerBin.js).

But there is no corresponding options in `relay-compiler` which would make this possible. And the only way to do so seems to be to write a custom plugin.