Support author for items

[witjem]

Context
RSS and Atom items has author field. It will be good if service will be support it.

Proposal
Add optional field author to feeds.yml config

matchers:
  engine: xpath
  author: # Optional props.
    name:
      expr: //meta[@name='author']/@content
    email:
      expr: //meta[@name='author']/@content

Should ignore author on the RSS if author name not found.

Reminder
Updates docs and examples too.