tools: add option to use custom template with js2c.py

This PR allows for embedders to to use custom template with js2c.py, therefore allowing non-UTF-8 for js2c in compiled-in-JS source files. The -t flag is chosen as it was the simplest unique flag name indicating TEMPLATE, but if there is a preferred more descriptive name scheme it can be changed!

Example usage by Electron here.

Checklist

• make -j4 test passes
• commit message follows commit guidelines