Add map value source

[dearchap]

What type of PR is this?

(REQUIRED)

• documentation
• feature

What this PR does / why we need it:

(REQUIRED)

In order to make the cli-altsrc more modular in terms on its provider it seems to be better if map value source can be incorporated directly into cli.

Which issue(s) this PR fixes:

(REQUIRED)

Special notes for your reviewer:

(fill-in or delete this section)

Testing

(fill-in or delete this section)

go test -run=TestMapValueSource

Release Notes

(REQUIRED)

New value source for maps has been included. This allows plugin providers for other formats(json, yaml, etc) to utilize base functionality instead of each source do custom nested lookups

[abitrolly]

Is it the "map value" or the map of value sources?

For debugging and bug reports it might be useful to have a structure and output that shows where option is sets, and which value is selected finally. Like:

$ CLI_PATH=/eee cli --path=/ccc --debug
urfave/cli: --path (default: none, env: /eee, cmdline: /ccc, final: /ccc)

[dearchap]

Is it the "map value" or the map of value sources?

For debugging and bug reports it might be useful to have a structure and output that shows where option is sets, and which value is selected finally. Like:

$ CLI_PATH=/eee cli --path=/ccc --debug
urfave/cli: --path (default: none, env: /eee, cmdline: /ccc, final: /ccc)

It is a map-value source i.e a source which can lookup values based on key.