Add handling for post-install and pre-uninstall scripts.

[freakboy3742]

Adds extension points to allow for the injection of a post-install or pre-uninstall script, and for options in the installer.

The value of the pre/post install script doesn't matter for the template; it's treated as a boolean.

The installer options is a list of dictionaries, with 4 keys of significance:

• name: the identifier that is used to
• title: A human label for the option
• description: A short description of the option
• default: true/false (optional; defaults false)

If the configuration defines an option with a name of foo, the OPTION_FOO property can be used to programmatically drive the installer.

Requires the use of the code from beeware/briefcase#2496.

briefcase-repo: https://github.com/freakboy3742/briefcase
briefcase-ref: pre-post-script

Refs beeware/briefcase-windows-app-template#73

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct