Support customizing the code skeleton

[yujqiao]

🚀 Feature Proposal

Add an option containing a string, which will be prepended before the auto-generated code snippet.

Motivation

Take C++ for example. vscode will lint the code and emit errors on types like vector defined in std namespace. This is the result of missing headers. The current auto-generated code snippet doesn't contain the necessary headers for IDE. So adding an option to prepend a custom snippet would be handy.

Contribution

Please describe the solution if you'd like to submit a PR.