config: read config from stdin

[ear7h]

This feature is helpful for running configs generated from another command (like dhall-toml 👀).

The - is an idiomatic alias for stdin used in commands like curl and grep.

[coveralls]

Pull Request Test Coverage Report for Build 9309a3b32-PR-802

• 2 of 5 (40.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.007%) to 45.025%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
config/config.go	0	3	0.0%
<!--	Total:	2	5	40.0%	-->

Totals
Change from base Build e444ac0b8:	-0.007%
Covered Lines:	5435
Relevant Lines:	12071

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💛 - Coveralls

[gdey]

@ear7h will review after #801 on Friday if I have time left.

[ARolek]

@ear7h can you rebase this with v0.14.x? LGTM otherwise.

[ear7h]

@ARolek done, and I improved the cli help text to help feature discovery