Add StringMap flag for altsrc

My urfave/cli version is

( Put the version of urfave/cli that you are using here ) 2.x

Checklist

• [x ] Are you running the latest v2 release? The list of releases is here.
• [ x] Did you check the manual for your release? The v2 manual is here
• [ x] Did you perform a search about this problem? Here's the GitHub guide about searching.

Dependency Management

• My project is using go modules.
• My project is using vendoring.
• My project is automatically downloading the latest version.
• I am unsure of what my dependency management setup is.

Describe the bug

Extend #1580 for altsrc flags

To reproduce

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Observed behavior

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Expected behavior

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Additional context

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Want to fix this yourself?

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Run go version and paste its output here

# paste `go version` output in here

Run go env and paste its output here

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