[FEATURE] Add xtra keyword argument or function call to MiniDataAPI Spec

[pydanny]

Is your feature request related to a problem? Please describe.

The .xtra control in the MiniDataAPI spec is powerful, let's explore adding it to the spec.

Describe the solution you'd like

• Document how the implementation should behave
• Implement in fastlite
• Document in the MiniDataAPI spec

Example code

Here's two ideas

1. As a keyword argument:

todos(xtra={'name': 'Danny'})

2. As a chainable function call:

todos(where="status='open'").xtra(name='Danny')

Similar implementations

Existing .xtra implementation is here

Problems solved

1. Encourages safer data control practices
2. That .xtra is immutable once set makes per-function an improvement to developer UX

Confirmation
Please confirm the following:

• I have checked the existing issues and pull requests to ensure this feature hasn't been requested before.
• I have read the project's documentation to ensure this feature doesn't already exist.

[jph00]

It should be a keyword argument. Search the fastlite source for anywhere we use self.xtra now, and have each of those functions take an xtra param. Then, in the function, do if not xtra: xtra=self.xtra, and then replace the usage of self.xtra in the function with xtra. Make sense?

We should implement it and use it first and see if we like it, before we document it.