GH-502: add support for pubmed ELMo model

[stefan-it]

Hi,

this PR adds support for the recently released PubMed model for ELMo.

Example usage:

from flair.data import Sentence
from flair.embeddings import ELMoEmbeddings

sentence = Sentence("Diabetes insipidus : The other diabetes")
embeddings = ELMoEmbeddings('pubmed')

embeddings.embed(sentence)

for token in sentence.tokens:
  print(token.get_embeddings())

Closes #502

[alanakbik]

This is great, thanks!

[alanakbik]

👍

[gokyildirim]

👍