Boolean fields in D8

[jonathanjfshaw]

It would be nice to do

Given an "article" content:
| title | promote |
| test  | Yes         |

instead of

Given an "article" content:
| title | promote |
| test  | 1            |

Using numeric values for booleans does not seem good BDD.
#44 seems to address the issue for D7; looks like we need a BooleanHandler for D8.

My suggestion is to try to use the field's 'off_label' and 'on_label' settings; if that fails then try true/false, 0/1, one/off, yes/no. If the value does not match any of these, throw an exception.

Doing this in a translation-sensitive way will be the clever part.