Added `override_schema/fields` to the `load` processor

[roll]

@akariv Please take a look. It's an initial try to implement this one - https://github.com/BCODMO/frictionless-usecases/issues/6#issuecomment-496394992

I'm not sure what the best naming solution:

• current override_schema/fields
• just schema/fields
• some combined schema (could be confusing though)

PS. It seems we don't need to add anything to DPP to propagate this change - https://github.com/frictionlessdata/datapackage-pipelines/blob/master/datapackage_pipelines/lib/load.py

[roll]

cc @cschloer

[coveralls]

Pull Request Test Coverage Report for Build 337

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 84.315%

---

Totals
Change from base Build 332:	0.08%
Covered Lines:	1575
Relevant Lines:	1868

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 336

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 84.315%

---

Totals
Change from base Build 332:	0.08%
Covered Lines:	1575
Relevant Lines:	1868

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 337

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 84.315%

---

Totals
Change from base Build 332:	0.08%
Covered Lines:	1575
Relevant Lines:	1868

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 336

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 84.315%

---

Totals
Change from base Build 332:	0.08%
Covered Lines:	1575
Relevant Lines:	1868

---

💛 - Coveralls