Support aliases in .yml files (altsrc.InitConfigFileInputSourceFunc)

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Thank you for this project. It's fantastic. I've been a long time user and I'm very happy.

I just added the ability to use the Aliases in my yml file when loading through altsrc (see binwiederhier/ntfy@91594fa#diff-d1253876bc7a1b30f67245948f61e61430af2f3f0bf72780e3c0314a24d152deR43-R50). I was wondering if that was something you'd be interested in adding as a feature.

If not, I'm alright too and you can close this ticket.

Example:

altsrc.NewStringFlag(&cli.StringFlag{Name: "listen-http", Aliases: []string{"listen_http", "l"},  ...}),

Should be loadable via the .yml file:

# Currently supported:
listen-http: ":80"

# Would also be supported with the suggested change (see Aliases above):
listen_http: ":80"
l: ":80"