Add support for stop limit orders in Bitfinex

[jhonabreul]

Description

Add support for stop limit orders in Bitfinex.

Related Issue

Related QuantConnect/Lean.Brokerages.Bitfinex#20
Closes QuantConnect/Lean.Brokerages.Bitfinex#21

Motivation and Context

Requires Documentation Change

How Has This Been Tested?

Unit tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves implementation)
• Performance (non-breaking change which improves performance. Please add associated performance test and results)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-functional change (xml comments/documentation/etc)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>