Don't prefix social mail link with "mailto:" if it is link to a URL rather than an email address.

Added the casper_social_link() function which can be used to retrieve social links via get_theme_mod().

If checks wether the link is an email and prefixes with "mailto:".

It purely checks if the link begins with "http" and if so assumes it's a link rather than an email.

There is a WordPress is_email() function but I think it may be unnecessarily heavy validation for this purpose.