allow printing to stdout if no output file is specified

[kwasnick]

It is desirable to allow printing of generated code to stdout rather than directly to a file. Printing to stdout makes it easier to integrate circuitgen with build systems such as Bazel or Gradle that have built-in directives and rules for redirecting command output into new files as part of their build step. Printing to stdout is a standard feature of most code generation tools. See destination flag of GoMock or the - argument of Counterfeiter. This change is backwards compatible with existing circuitgen usage.

See #12