Add option to add phone links for mobile use

This is a good idea, but it needs work.

• If I use 555-555-1234 and add a link to a phone number, it makes the link tel:+555-555-1234. Phone numbers without a country code should not have a + in front.
• If I enter tel:555-555-1234, it still matches, but clicking the Phone match will make the link tel:+tel:555-555-1234, which is definitely broken. This can be fixed with:

    // Strip the tel: prefix to match only the phone part of the string.
    $string = str_replace('tel:', '', $string);