honour increment config value for Mainline

[JonCubed]

Adds support to Mainline mode to honour Increment value in config. Previously if not a tag or commit message bump would default to Patch increment.

Includes:

• Added new method to IncrementStrategyFinder to work out default Increment Strategy for a branch
• Updated MainlineDevelopmentMode to use new method instead of defaulting to Patch
• Added unit test to cover setting increment in config

Fixes #1295

[asbjornu]

LGTM! Thanks!