Add `forceNewlineBetweenStepBlocks` option

[jdeniau]

Do not force blank lines between steps block by default, but give the possibility to do so with a configuration option.

Given the following block

Given foo
When bar

Before this modification, the output is:

Given foo

When bar

With the new default option, the output will be:

Given foo
When bar

If the input does contain a blank line (or more), the output will keep the blank line:

# input
Given foo

When bar

# output
Given foo

When bar

Fixes #15

TODO

• [x] test both options
• [x] activate option by default