Add StringMap flag for altsrc

[dearchap]

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

Describe the steps or code required to reproduce the behavior

Observed behavior

What did you see happen immediately after the reproduction steps
above?

Expected behavior

What would you have expected to happen immediately after the
reproduction steps above?

Additional context

Add any other context about the problem here.

If the issue relates to a specific open source GitHub repo, please
link that repo here.

If you can reproduce this issue with a public CI system, please
link a failing build here.

Want to fix this yourself?

We'd love to have more contributors on this project! If the fix for
this bug is easily explained and very small, feel free to create a
pull request for it.

Run go version and paste its output here

# paste `go version` output in here

Run go env and paste its output here

# paste `go env` output in here