set default threads to 1

[AndreyNikiforov]

What: sets default --threads-num parameter to 1
Why: multi-threaded download causing issues when duplicate files exist in iCloud or on disk. Changing default will help reduce a number of issues. #155
Note: Discussion about deprecating multi-threaded downloading is separate from this fix

[coveralls]

Pull Request Test Coverage Report for Build 179

• 0 of 0 changed or added relevant lines in 0 files 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 178:	0.0%
Covered Lines:	473
Relevant Lines:	473

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💛 - Coveralls

[menkej]

Actially I was thinking about the same. Change is simple, side effects not expected.