Migrations: Add PowerShell commands

This is a first draft of the migrations PMC commands. They should work fine for a single-context project (there is currently no way to specify the migrations directory). They also need more polishing (including logging and documentation). The following commands more-or-less work.

- `Add-Migration` - Works as before without the `-Force`, `-StartUpProjectName`, `-ConnectionString` (and related), `-IgnoreChanges`, and `-AppDomainBaseDirectory` parameters yet.
- `Update-Database` - Works as before without the `-SourceMigration` parameter yet
- `Use-EFContext` - (new) Sets the default DbContext to use (doesn't validate it actually exists yet)
Can the args dictionary be empty, does this need to handle missing arguments, or will that be done later?