Restrict parse_int radix to [2, 36]

[ning-y]

Behaviour now follows ES6 except:

• If either args str or radix is not provided, source error is thrown
• If first arg is not str, source error is thrown
• If second arg is not integer in [2, 36], source error is thrown

Resolves #52.

[coveralls]

Pull Request Test Coverage Report for Build 159

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.1%) to 61.502%

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Totals
Change from base Build 153:	1.1%
Covered Lines:	798
Relevant Lines:	1231

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💛 - Coveralls

[ning-y]

@martin-henz Are the specifications here reasonable? Instead of returning NaN for values of radix outside of [2, 36], we throw an error. This is because ES6 specifies the radix to be an argument between 2 and 36. In addition, we throw an error for non-integer radix arguments. Nonetheless, everything that passes without an error in source returns the same thing that ES6 would.

[martin-henz]

Yes, that's fine.

[martin-henz]

Yes, that's fine.