update strong and em substitution routine with code friendly regexp.

Update for `_do_italics_and_bold` subroutine with code friendly conversion to address https://sprint.ly/product/1/#!/item/7274. 

A unit test, `footnotes_underscores`, is updated so that it respect code friendly conversion. Additional unit test are added in `sprint.ly` repo itself, since these unit test are not interest of markdown2 itself.

Note: This does not use `code_friendly` extra feature of markdown2, since it will disable common use case of em and strong.