Generate list of destinations

[umputun]

see #104

This PR allows exporting the list of destinations to either json (default format) or any custom format defined by a user-provided template file (go templates format). The generated list can be sent to stdout (by default) or a custom file.

@knutov - I have added this thing even with the templates because I don't want to abuse users by forcing them to use jq ;)

I still think this feature is a little bit exotic, but probably for users migrating from ansible it will make more sense.

[coveralls]

Pull Request Test Coverage Report for Build 5073341063

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 43 of 59 (72.88%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 84.586%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/spot/main.go	24	32	75.0%
pkg/runner/runner.go	19	27	70.37%

Totals
Change from base Build 5048375418:	-0.3%
Covered Lines:	2217
Relevant Lines:	2621

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💛 - Coveralls