Preprocess (populate, generate) files with the content of other files

Question

I'm going to explain what my problem is using a representative example.

Let's say I have these 2 configuration files:

# product-conf.file
seo_title: general_title
seo_description: seo_description
seo_canonical: seo_canonical

product_id: general_id
title: general_title
intro: general_intro

.

# service-conf.file
seo_title: general_title
seo_description: seo_description
seo_canonical: seo_canonical

service_id: general_id
title: general_title
products: list_products

As you can see, the 3 first lines (configuration fields) are exactly the same. I'm actually using YAML for these files. I would like to have pieces of code in maintainable files and include them with calls. I need a preprocessor for these. Something like:

# snippet-seo.file
seo_title: general_title
seo_description: seo_description
seo_canonical: seo_canonical

.

# product-conf-master.file

@include snippet-seo

product_id: general_id
title: general_title
intro: general_intro

.

# service-conf-master.file

@include snippet-seo

service_id: general_id
title: general_title
products: list_products

The preprocessor will read all the master files in /masters/*, attend all the calls and substituting them for the appropriate snippet from /snippets/ and saving the result in /

I'm doing the call with @ but I can choose whatever other format is suitable for the chosen preprocessor. I used this way because it strongly resembles to the SASS directive @extend or @include.

What is the best and easiest way I can achieve this? A package for node would be my first choice.