Have Suggest extensible

[davidTurczi]

Elastic supports completion suggestion as an experimental feature. I understand the point of not building experimental features into the client, but nonetheless I'd like to have the ability to extend this way myself.

SuggestDescriptorBucket has a fixed TermSuggest and PhraseSuggest property, please change it into a dictionary-like wrapper, so I can add ANY descriptor inheriting from BaseSuggestDescriptor (so I could implement my own CompletionSuggestDescriptor)

[gmarz]

@davidTurczi

Completion suggestions were actually added in this PR: #376 and will probably be available in the next NuGeT release.

[davidTurczi]

Ah that's great! I'll go instead and ask for #395 a new nuget :)