Expose custom transformers when emitting

[dsherret]

Would be nice to do this:

project.emit({
    customTransformers: {
        // optional transformers to evaluate before built in .js transformations
        before: [context => sourceFile => visitSourceFile(sourceFile, context, numericLiteralToStringLiteral)],
        // optional transformers to evaluate after built in .js transformations
        after: [],
        // optional transformers to evaluate after built in .d.ts transformations
        afterDeclarations: []
    }
});

function visitSourceFile(sourceFile: ts.SourceFile, context: ts.TransformationContext, visitNode: (node: ts.Node) => ts.Node) {
    return visitNodeAndChildren(sourceFile) as ts.SourceFile;

    function visitNodeAndChildren(node: ts.Node): ts.Node {
        return ts.visitEachChild(visitNode(node), visitNodeAndChildren, context);
    }
}

function numericLiteralToStringLiteral(node: ts.Node) {
    if (ts.isNumericLiteral(node))
        return ts.createStringLiteral(node.text);
    return node;
}