Add option emptyLines

[zentby]

Solves #242

It introduces a new option emptyLines for default and full version.

When it is enabled, it will preserve the empty lines and convert them into <p></p>.

e.g.
Markdown

line1


line2

Before:

<p>line1</p>
<p>line2</p>

After:

<p>line1</p>
<p></p>
<p>line2</p>

Note there will be an empty line not being preserved, to be compatible for existing line terminator.

[coveralls]

Coverage increased (+0.02%) to 98.569% when pulling fcc4a2f on zentby:empty_lines into 58b6945 on jonschlinkert:master.

[Dagur]

A copule of comments:

• Doesn't work if you use "commonmark" instead of "full"
• README needs to be updated