Make template content types regular classes not case classes, chg + op to +=.


Currently pattern matching over template content types is deceiving, only the initial value is capture in the pattern matching, while the toString() captures subsequently appended content.  Also the + operator should not be used for mutation because it breaks a strong convention: this patch adds a += operator (replacing + in Appendable trait), and deprecates the + operator.

Lighthouse issue # 412:  https://play.lighthouseapp.com/projects/82401/tickets/412-html-case-class-mutable-causing-erronous-pattern-matching