Load Pipeline with YAML config file

[tanaysoni]

This PR adds support to load a Pipeline with a YAML configuration file.

Here's a sample Pipeline YAML:

version: '0.7'

components:    # define all the building-blocks for Pipeline
  - name: MyReader       # custom-name for the component; helpful for visualization & debugging
    type: FARMReader    # Haystack Class name for the component
    params:
      no_ans_boost: -10
      model_name_or_path: deepset/roberta-base-squad2
  - name: MyESRetriever
    type: ElasticsearchRetriever
    params:
      document_store: MyDocumentStore    # params can reference other components defined in the YAML
      custom_query: null
  - name: MyDocumentStore
    type: ElasticsearchDocumentStore
    params:
      index: haystack_test

pipelines:    # multiple Pipelines can be defined using the components from above
  - name: my_query_pipeline    # a simple extractive-qa Pipeline
    nodes:
      - name: MyESRetriever
        inputs: [Query]
      - name: MyReader
        inputs: [MyESRetriever]

To load a Pipeline:

from pathlib import Path
from haystack import Pipeline

pipeline = Pipeline.load_from_yaml(Path("sample.yaml"))
prediction = pipeline.run(query="What is the capital of Germany?", top_k_retriever=10, top_k_reader=3)

[tholor]

Looking very good already. Added a few smaller comments ...

[tholor]

Ready to merge after formatting the code snipped properly (see comment)