Refactor action_acm_tag()

[GaryJones]

• Extracts the logic for getting the output HTML into a new get_acm_tag() method.
• The action_acm_tag() method now just echoes the result of a call to get_acm_tag(). It no longer supports an $echo parameter.
• The shortcode() method now calls and returns get_acm_tag(), instead of action_acm_tag() with a false echo parameter.

Closes #94.