Allow templates on script interpreter

[ryanxcharles]

It is possible the script interpreter is faulty. In fact, any consensus-critical code may be faulty. This could result in lost funds if someone relied on faulty consensus code that put them on a fork. One way to address this issue is through the use of templates. Allow the script interpreter and other consensus-critical code to be disabled for non-standard transactions. Allow a whitelist of template scripts to constitute standard transactions (similar to or perhaps idential to IsStandard in bitcoin core).

This idea comes from Peter Todd: https://twitter.com/petertoddbtc/status/541018246858960897