Handle `.gitignore` when publishing package

[ivankravets]

If the export field is not declared in the package manifest, read filters from .gitignore.

[nomis]

PlatformIO doesn't just use .gitignore in this scenario, it also enables lots of built-in rules including the exclusion of my docs directory.

[ivankravets]

Yes, this is the default behavior. You can manually control what to export.