Pipelines: add PoS support
## 🚀 Feature

As `Pipelines` were recently added for many tasks including NER, Sentiment Analysis, it'd be great to also enable Part-of-Speech tagging.

## Motivation

PoS tagging is a very useful task, and often used as an evaluating downstream task for new models.

## Additional context

Current available tasks for `Pipelines` are described [here](https://github.com/huggingface/transformers/blob/master/src/transformers/pipelines.py#L831).

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>We now have a more general `TokenClassificationPipeline`, @arnaudmiribel (this is just an alias to the `NerPipeline`)