Check if SIGNATURE_FILE_NAME exists and use it as output_filename

[Luca1991]

Hi,
I've already discussed this PR with @psifertex on Slack.
Long story short: I'm working on a script to automatically create signatures from .LIB files (more info here: #28) and this PR basically allows me to set the filename for the resulting .SIG directly from my script.

Thanks,
Luca

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[psifertex]

lgtm

[Luca1991]

@psifertex do you know when it will be possible to download the new version containing this PR from the official plugin repository? Thanks a lot.

[psifertex]

It should be live now.