Add mandatory option; e.g require element for row to be valid

[stooit]

Fixes #9

As per added docs:

Mandatory element

Some elements may be considered mandatory for a row to be considered valid. For example; if a page does not contain a 'Title' then it may fail a mandatory requirement and be skipped.

This is controlled via the mandatory option against a field. For example:

mappings:
  -
    field: title
    selector: '#content-main h1'
    type: text
    options:
      mandatory: true