enhance(get): Allow setting a default value on Config::get()

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[coveralls]

Pull Request Test Coverage Report for Build 51

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 47:	0.0%
Covered Lines:	19
Relevant Lines:	19

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💛 - Coveralls

[austinpray]

added some failing test cases 5244479

[tangrufus]

Discussion: If Config::get() returns default values, do we still need UndefinedConfigKeyException?

In my opinion, since we always have a default value ($default = null), we never need to throw UndefinedConfigKeyException.
If users really need to check whether a key is defined in the configMap, we can add a Config::has() method.

I vote for something similar to https://github.com/laravel/framework/blob/4a74d20f8d960b38d75dd525946e893c75a94dbc/src/Illuminate/Config/Repository.php#L33-L58

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But, see

wp-config/docs/why.md

Line 127 in 629c533

I throw a `UndefinedConfigKeyException` when a user attempts to fetch a value with `Config::get(k)` that doesn't exist in `Config::$configMap`. I do this because the control flow is nice and `return null` is not working because null is a viable configuration value. We don't use `Config::get(k)` anywhere but it is a part of the API because I can see users needing it.