Added API to get Verilog from Chisel (#676)

* Added API to get Verilog from Chisel

* Removed second emitVerilog implementation, added scaladoc

Author: azidar

src/main/scala/chisel3/Driver.scala

@@ -95,6 +95,18 @@ object Driver extends BackendCompilationUtilities {
 
   def emit[T <: RawModule](ir: Circuit): String = Emitter.emit(ir)
 
+  /** Elaborates the Module specified in the gen function into Verilog
+    *
+    *  @param gen a function that creates a Module hierarchy
+    *  @return the resulting String containing the design in Verilog
+    */
+  def emitVerilog[T <: RawModule](gen: => T): String = {
+    execute(Array[String](), { () => gen }) match {
+      case ChiselExecutionSuccess(_, _, Some(firrtl.FirrtlExecutionSuccess(_, verilog))) => verilog
+      case _ => sys.error("Cannot get Verilog!")
+    }
+  }
+
   def dumpFirrtl(ir: Circuit, optName: Option[File]): File = {
     val f = optName.getOrElse(new File(ir.name + ".fir"))
     val w = new FileWriter(f)