Add plugin hook in schema.php to allow localizing default options

[RanYanivHartstein]

The populate_options() function in schema.php sets several default options that are locale-specific, such as the first day of the week, feed language and possibly timezone (depending on your locale). Adding a plugin hook to populate_options() will allow overriding these defaults in the locale files.

I included a patch to add a plugin hook to populate_options(), I tested it with 2.5.1

Patch to add plugin hook in populate_options()

Technically, you could do this via the pre_ hooks for add/update, but it'd be a lot more work.

(In [7848]) Add populate_options action. Props RanYanivHartstein. fixes #6854