Make `strict` behaviour consistent and document it

Various cucumber implementations have inconsistent behaviour for `strict`:

* Default value true or false?
* Does strict cause undefined steps to fail?
* Does strict cause pending steps to fail?

I propose we make `strict` default to `true`. By defaulting to true we're actively discouraging users from making a lot of WIP, and I think an opinionated tool like Cucumber should do stuff like that.

I also propose the following behaviour:

| | strict=true | strict=false |
|----------------|-------------|--------------|
| undefined step | error | no error |
| pending step | error | no error |
| failed step | error | error |
| passed step | no error | no error |
| ambiguous step | error | error |

I haven't looked into whether the various implementations behave like this. Let's agree on the desired behaviour - then create issues for the implementations that deviate so we can fix this.

When all implementations are fixed we should put this up on the docs site.