Allow date placeholders in result sets

[Ch0ronomato]

Allow for the following Then clause:

Then the result exactly matches with date placeholders
| foo | date_bar  |
| 1     | 1 day ago |

[Ch0ronomato]

As a note, the work around for this is using Then the result includes or just not including the date field in the table.

[moertel]

In order to keep support for adding explanatory text at the end, I changed the order for the with date placeholders keyphrase. You can now use this as follows:

• Bare matcher:

Then the result exactly matches:
  | foo | date_bar   |
  | 1   | 2017-01-01 |

• Matcher with placeholder:

Then the result with date placeholders exactly matches:
  | foo | date_bar  |
  | 1   | 1 day ago |

• Matcher with placeholder and explanatory text:

Then the result with date placeholders exactly matches some expected state:
  | foo | date_bar  |
  | 1   | 1 day ago |

Placeholders have been added for all result matchers.