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)
We ensure it's added up the callstack (in the PowerShell script), but this class is public... I'll add a TODO to consider validating this and other arguments.