Filter out unsupported packages

[maraf]

Changes

• Update reference packages for tests to require dependency on GitExtensions.Extensibility in various versions.
• Add required 0.2.0 version in tests.
• Add required 0.2.0 version in Git Extensions plugin.

Summary of nuget version ranges

• Plugin Manager now requires 0.2.0.
• Plugin author must publish a nuget package with dependency on GitExtensions.Extensibility with version range satisfying 0.2.0, examples:
  • [0.2.0] - plugin requires concrete single version.
  • [0.2.0,0.3.0) - like "Visual Studio extensions version" (0.2.0 <= plugin < 0.3.0)

Fixes #61.