Add a way of running a script as part of creating the template

[7sharp9]

Currently theres no way (afaik) to runa script when a template is created, so I cant install npm / other dependencies for example.

[rmunn]

An extra parameter in templates.json seems like the natural place to specify this, which could specify the script file to run, and any command-line parameters to pass to the file. To ensure cross-platform compatibility, we'd need to be able to specify a different script per platform. We could take some inspiration from how VS Code handles the tasks.json file.

[rmunn]

Or perhaps cross-platform compatibility could be ensured by having the template-creation script be a Javascript file, specified in the templates.json, which the generator could require and then run a specified entry-point function.