Pass context to database drivers

[miagilepner]

This PR adds support for passing a context to database drivers. I've added migrate.NewWithContext() and migrate.NewWithSourceContext() which accept a context. For each driver, I've added OpenWithContext and WithInstanceContext which will pass the context to the database during any initial connection. This handles https://github.com/golang-migrate/migrate/issues/571.

[coveralls]

Pull Request Test Coverage Report for Build 1359990807

• 228 of 275 (82.91%) changed or added relevant lines in 18 files are covered.
• 7 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+1.04%) to 58.734%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
database/cassandra/cassandra.go	8	10	80.0%
database/firebird/firebird.go	9	11	81.82%
database/mysql/mysql.go	10	12	83.33%
database/spanner/spanner.go	10	12	83.33%
database/sqlserver/sqlserver.go	11	13	84.62%
database/cockroachdb/cockroachdb.go	14	17	82.35%
database/redshift/redshift.go	10	13	76.92%
database/stub/stub.go	11	26	42.31%
migrate.go	32	48	66.67%
<!--	Total:	228	275

Files with Coverage Reduction	New Missed Lines	%
database/cassandra/cassandra.go	1	47.37%
database/cockroachdb/cockroachdb.go	1	62.98%
database/firebird/firebird.go	1	75.58%
database/mysql/mysql.go	1	64.74%
database/redshift/redshift.go	1	67.86%
database/spanner/spanner.go	1	69.51%
database/sqlserver/sqlserver.go	1	70.08%
<!--	Total:	7

Totals
Change from base Build 1346768578:	1.04%
Covered Lines:	3880
Relevant Lines:	6606

---

💛 - Coveralls

[iwarapter]

hey i'd love to see this change merged are their any blockers (apart from it being out of date with master)?

[dhui]

Thanks for the PR!

hey i'd love to see this change merged are their any blockers (apart from it being out of date with master)?

Sorry I need to find time to review this :( I'll try to get to it during the holidays.