mysql type overrides

[cmoog]

Fixes #273 .

This implements package-level and global type overrides for mysql usage.

Implementation notes

I adjusted a few data structures to ease the passing of package settings and database schema throughout the parsing and generation process.
The primary new structures are shown below.

type PackageGenerator struct {
	*Schema
	dinosql.CombinedSettings
	packageName string
}

type Result struct {
	PackageGenerator
	Queries []*Query
}

By separating the schema and package settings from the constructed queries, we're able to keep many of the parsing functions more functionally pure.

[kyleconroy]

This is looking very close to shipping. I left a few comments that should be easy to address. After this is merged, I'll work on porting most of the MySQL tests to the new end-to-end framework.

[kyleconroy]

The booktest examples is taken from xo/xo. The PostgreSQL and MySQL versions should be the same as that project.

Can you back out your multiple changes to this file and add tests for overrides in the internal/endtoend/testdata directory?

[cmoog]

@kyleconroy How do you think we should organize the new testdata directory with multiple database engines?

[kyleconroy]

Individual tests should be in a subdirectory of testdata. Unless we're testing multi-engine configuration, we probably want to keep tests for each engine separate.