Minitrade

- [Fix] Handle missing field in IB API response.

- [Fix] Resolve issue with backtesting on Windows. Run `minitrade init` to complete setup before use.

- [New] Redesign weight allocation API to be more expressive in defining strategies.

- [Fix] Fix notification formatting.

- [New] Add `Strategy.prepare_data()` to suppport strategy specific data provider.
- [New] Add `Strategy.start_on_day()` to control how many data bars should be initially available in `next()`.
- [New] Add UnionQuoteSource that supports getting data from different sources.
- [New] Allow columns other than OHLCV in data for backtesting.
- [New] Indicator plot can be turned off using by `plot_indicator=False`.
- [New] Better handling of very large asset space, e.g. truncating data in display/charting.
- [Change] "Volume" is now an optional column in data for backtesting.

- [Fix] Fix QuoteSource may return data not in ascending time order in some cases.