Update BIP 141 to include definition of Virtual transaction size and Transaction weight

Virtual transaction size and Transaction weight are not consensus-critical, but are used in bitcoin-core IsStandard() policy, and v0.13.0+, RPCs calls return virtual transaction size instead of serialized transaction size. Although non-normative, BIP 141 should include definitions for these concepts. https://github.com/bitcoin/bitcoin/pull/8747 is a PR to fix the RPC help text to refer to virtual transaction size instead of transaction size.

Specification