Rename "boostrap" to "migrate" mode

[jcjones]

From #4 (comment)

Basically the "bootstrap" mode is a specialized pt-online-schema-change that needs handholding. It does bootstrap the partition map from nothing, but maybe it's easier to describe it as migrating, and in doing so split the --in and --out forms of it to two different subcommands: migration-bootstrap and migration-generate-plan perhaps