Support for partial templating

[ccremer]

There are use cases where a repository may need to customize a bigger part of a template, but keep everything managed by modulesync.
Those files would then be merged toghether.

Example 1: Add custom job to a GitHub workflow. One could provide such YAML via .sync.yml file, but maintaining GitHub actions in this file is not really the way to go.

Possible implementation:

• With Go templating: {{ include "custom-file.txt" "fallback-text" }}