cmd: Allowing to change the output file name from wire_gen.go to other

[josgilmo]

Issue #192

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Please edit the title of this pull request with the name of the affected component, or "all", followed by a colon, followed by a short summary of the change.

[vangent]

Did you see the comment on the issue about wire possibly generating multiple files?
#192 (comment)

Currently it only generates one, but that may change later, e.g., generating a file for tests.
In that scenario, a single output file name will not work well. The suggestion in the bug is
to provide an output file prefix (default empty string). Does that work for your use case?