Add load infer, cast and error strategies

[akariv]

This PR generalizes the way that load infers and casts data types, deprecating the validate and force_strings arguments into three 'strategy' arguments: infer_strategy, cast_strategy and on_error (to decide what to do when validations errors occur).

See here for usage:
https://github.com/datahq/dataflows/pull/88/files#diff-0edecdf275b79739bdb4ca088cfb67e5R67

[coveralls]

Pull Request Test Coverage Report for Build 304

• 35 of 40 (87.5%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 83.944%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dataflows/processors/load.py	34	39	87.18%

Totals
Change from base Build 303:	-0.01%
Covered Lines:	1558
Relevant Lines:	1856

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 309

• 42 of 42 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 84.222%

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Totals
Change from base Build 303:	0.3%
Covered Lines:	1564
Relevant Lines:	1857

---

💛 - Coveralls

[rufuspollock]

@akariv what issue does this relate to? What is it adding? Wasn't super clearr.

[akariv]

@rufuspollock updated description of the PR, thanks :) !