NER: Add new WNUT’17 example
Hi,

this PR extends the NER example section, and adds an extra section for fine-tuning a NER model on the (difficult) WNUT’17 shared task:

> The WNUT’17 shared task focuses on identifying unusual, previously-unseen entities in the context of emerging discussions.
> Named entities form the basis of many modern approaches to other tasks (like event clustering and summarization), but recall on
> them is a real problem in noisy text - even among annotators. This drop tends to be due to novel entities and surface forms.

I also added my pre-processing script, that splits longer sentences into smaller ones (once the max. subtoken length is reached).
Looks great!<|||||>(I've relaunched the failing CI test that's unrelated)<|||||>@julien-c do you think rebasing onto latest master would fix that problem?