Snowflake - Tests, Role, Warehouse and Multi-Statement

[ninthclowd]

Hello, as I mentioned in #404, I need this functionality for a project I am working on. This program is great; I really like the design.

This PR adds the following:

• Test coverage for snowflake using a SQL mock library, since it is not possible to run snowflake in a local container
• The ability to specify connection role through the x-role parameter
• The ability to specify connection warehouse through the x-warehouse parameter
• The ability to specify multi-statement support through the x-multi-statement parameter, which enables snowflake's built in multi-statement support
• Removes pq.* module references, since the pq library is not referenced by the gosnowflake library

I understand there are some older PRs out there that do similar functionality, so I understand if you would prefer to continue to see those through to the end and don't want to accept this PR. I'm only submitting it in the case that those contributors are no longer available, or if you would prefer the new functionality be covered by tests.

Thanks!

Addresses #543, #431 and #404

[coveralls]

Coverage increased (+1.0%) to 58.772% when pulling 0632654 on ninthclowd:feature/snowflake into 03613f1 on golang-migrate:master.