Add optional headers to configuration for scrape component

[dippysan]

Description:

Adds headers: configuration to allow
Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#6455

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: scrape
    resource: http://www.bom.gov.au/vic/forecasts/melbourne.shtml
    name: Melbourne Forecast Summary
    select: ".main .forecast p"
    value_template: '{{ value | truncate(255) }}'
    # Request every hour
    scan_interval: 3600
    headers:
      User-Agent: Mozilla/5.0

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[homeassistant]

Hi @dippysan,

It seems you haven't yet signed a CLA. Please do so here.

Once you do that we will be able to review and accept this pull request.

Thanks!

[fabaff]

Thanks 🐦