Fix:(issue_2225) Make external flags more robust.

[bbadour]

Use delegation to implement the Value+Flag paradigm used for other flag types.

Implement all of the Flag interfaces except Count.

Borrowed heavily from flag_bool.go and flag_generic.go

Add context to the error message when setting an external flag's value to its default value to avoid cryptic error messages like: "syntax error: expected file.go:234"

And suppress the error for odd-ball external flags that report the string representation of the zero-value of some structure as the default, but do not accept that string as input. Detect such odd-balls by their Get() method returning nil.

What type of PR is this?

• feature

What this PR does / why we need it:

Solve:

• cryptic error messages like "syntax error: expected flag.go:234"
• errors requiring external boolean flags have a value when used
• avoidable astonishment when AllowExtFlags flags behave differently from identical flags defined using urfave/cli

Which issue(s) this PR fixes:

Fixes #2225

Testing

Installed gfmrun and make passes. Also the same code passes the test cases for a large code base that uses the feature.

Release Notes

Improve support for `AllowExtFlags`