Use property alias for metas plugin

[oscarotero]

Enter your suggestions in details:

The metas plugin has the defaultPageData option to configure default values for the metas properties. I think this is not convenient due two reasons:

• It cannot be configured per page because it's defined in the _config.ts file
• If some defaults values are defined in a _data file (for example, metas.title), then you need to do metas.title = null to use the default value configured.

I propose to change the plugin to implement a syntax to define aliases to existing variables. For example:

title: Hello world
description: This is my site

metas:
  title: $title
  description: $description

Any value starting with $ sign would be interpreted as an alias to an existing value (in the example above, title and description. It could allow also values like $header.title.

This allows to have different defaults for different types of pages, using _data files in subfolders.

Edit:

Maybe the sign ~ is better, because it's what's used by the url plugin to refer to source files (like ~/about-me.md):

title: Hello world
description: This is my site

metas:
  title: ~title
  description: ~description

Other alternative syntaxes:

title: Hello world
description: This is my site

metas:
  title: &title
  description: &description

(Cons: The & sign is a thing in Yaml).

title: Hello world
description: This is my site

metas:
  title: {{ title }}
  description: {{ description }}

(Const: It looks like nunjucks, people may think it's possible to use filters and other nunjucks features).

Any feedback is welcome!