Added the ability for custom metas to be exposed in the pages object.

/*
Title: Sub Page
Custom: Me So Custom
*/

the `Custom` property can be used in the pages foreach loop as `{{ page.custom }}`.

I wanted this functionality because I wanted to assign each page a category/tag. All my pages have the property so they read fine in the template.

There is also the possibility of making pages have types, draft status etc.

Title is the only required tag. It throws an error when its missing.