Add manpages from source

[suntong]

Not quite like #339,

I need to add manpages from source.
I.e., I have app.1 in source format as oppose to app.1.gz.

Can nfpm somehow help turn app.1 into app.1.gz pls?

[suntong]

Try to create app.1.gz from app.1 in workflow steps prior to calling nfpm.