[WIP] - Adding a provider

[mfridman]

I don't intend to merge this branch, simply putting up a PR to show where my thinking is at.

This may be done in a backwards-compatible way, since the entry point to a well-defined struct is goose.NewProvider.

I'll go into more detail (eventually), but the summary is:

• Add a well-defined Provider struct that can be initizlied independently. No more global state, allowing multiple providers per runtime
• All methods that apply migrations now support context.Context
• Decouple dialect from driver name. I think the user should pass a *sql.DB and use whatever driver they want, they just have to match it to one of the supported dialects
• Move CLI concerns to the CLI, and move package concerns to the package. E.g., the goose library should not log so much as a library
• Add support for Go functions that accept *sql.DB
• ... add more

// GoMigration is a go migration func that is run within a transaction.
type GoMigration func(tx *sql.Tx) error

// GoMigrationNoTx is a go migration funt that is run outside a transaction.
type GoMigrationNoTx func(db *sql.DB) error