figure out proper handling of lists with some blank lines

[3b]

in lists with some items separated by blank lines, we currently treat all elements as paragraphs.

markdown.pl only treat entries before/after blank lines as paragraph (2,3,4 in example).

Github treats everything starting before the first line as a paragraph (2,3,4,5,6 in example below).

see http://babelmark.bobtfish.net/ for a comparison of various other implementations, all 3 behaviours seem reasonably common

test case:

* l0
* l1
* l2

* l3

* l4
* l5
* l6

test case as displayed by github:

• l0
• l1
• l2
• l3
• l4
• l5
• l6