Make it easier to run migrations as repeatable commands

[gggeek]

Since the migration definition language has acquired a lot of the bells and whistles of a full content manipulation language, it would be nice to be able to run specific migrations as standalone cli commands.

As such, those commands should:

• not be tracked in the db
• allow concurrent execution
• be easy to discover from the cli

Note: this might be implemented as a separate bundle

[gggeek]

As part of 5.8, there will be a --force flag available for the migrate commands. Not 100% there, but I would say 80%...

[gggeek]

Other improvements related to the topic:

• when executing migrations passing references values on the cli, save those in the mig. execution status line
• allow yaml migrations to include other yaml migrations. This way one could build a "library" of migration building blocks to be reused in different specific migrations (note that this defeats in large part the purpose of having a repeatable way of creating a known db state, in the case the 'lib' migrations do get altered after being executed once...)

[gggeek]

The two improvements listed above have been implemented in rel. 6.3.0