Add SystemVerilogCompiler class

This adds a SystemVerilogCompiler class that extends, without modifying,
the existing VerilogCompiler. This is used by FIRRTL's Driver and will
cause a warning to be emitted indicating that the SystemVerilogCompiler
behaves the same as the VerilogCompiler.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>