Add ability to provide defaults for custom options

[mbargull]

This adds to new keys (add_to_path_default, register_python_default) with which the installer's custom options can be given explicit default values in construct.yaml. Useful if one wants to create an installer which, by default, does not interfere with the system (i.e. both values set to False).

[astaric]

@ilanschnell is this change something that could be included in constructor?

I am currently trying to build win installer for a package that installs its own menu shortcuts and thus does not need to change default path. Personally, I would not mind setting the defaults and not showing the choice screen to users at all, but providing different defaults is a good start.

[msarahan]

Thanks for this helpful change, @mbargull. Merging.

[github-actions]

Hi there, thank you for your contribution!

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Please open a new issue or pull request if needed.

Thanks!