Proposition for LoremIpsum::each_paragraph

<code>each_paragraph</code> is essentially factoring-out some of <code>paragraphs</code>' functionality, so that there is better separation of responsibilities, etc. As a free side-effect, we get more flexibility if desired.

<code>each_paragraph</code> takes over the paragraph generation logic, while <code>paragraphs</code> takes over decoration and utility roles. It is essentially a convenience wrapper-method for <code>each_paragraph</code>.

<code>each_paragraph</code> honours the <code>:sentences</code> and <code>:random</code>-related options. <code>paragraphs</code> passes its options to <code>each_paragraph</code>, so they can be "forwarded".

This pull request builds on top of #70, which suggested changing the behaviour of <code>paragraphs</code> so that they yield their decorated paragraphs if a block is given. I raise this pull request as a separate matter but on top of #70 so that the differences of refactoring are clearer and so that it can be discussed separately. In case #70 is rejected, this refactoring is still valid and can be fixed to apply cleanly.