allow to specify encoding for load processor when loading from file

[OriHoch]

allows to bypass the character encoding detection in datapackage-py

[coveralls]

Pull Request Test Coverage Report for Build 200

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 78.082%

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Totals
Change from base Build 198:	0.03%
Covered Lines:	1140
Relevant Lines:	1460

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💛 - Coveralls