Allow settings to be saved by section rather than page

[NateWr]

See #3 (comment)

Currently all settings on a page are saved in a single row in the options table. It would be nice to be able to save settings of a specific section to their own row.

Thoughts on how to do this:

• Support an is_option argument for Sections. https://github.com/NateWr/simple-admin-pages/blob/master/classes/Library.class.php#L243
• Compute an option_name argument for Settings based on whether the Page or Section should be used. https://github.com/NateWr/simple-admin-pages/blob/master/classes/Library.class.php#L268
• Use the option_name argument when outputting the input name for a Setting. https://github.com/NateWr/simple-admin-pages/blob/master/classes/AdminPageSetting.class.php#L251-L258

[NateWr]

This should be considered in the context of #13 and the REST API, for which it might be advantageous to be able to offer more machine-readable access to individual settings.