feat(email-tag): Add meta data email tag

[ravinderk]

@DevinWalker @mathetos some time user ask for email tag which does not exist in core. Instead of asking them to wait until new release we can point this snippet which helps to render donation metadata into an email.

Shortcode format: {meta_*}
For example: {meta__give_payment_transaction_id}, {meta__give_payment_donor_email}

Note: meta key must contain string instead of array