Fold content on the same line as the heading

As suggested by @vovkkk:
https://github.com/demon386/SmartMarkdown/issues/12#issuecomment-15336318
Improved handling of Ctrl/Cmd+Enter at folded headings
simplify syntax
Merge pull request #4 from vovkkk/patch-4

simplify syntax
adds class SmartNewLineCommand

fix issue when folded headline is last one  in file
fixes class SmartNewLineCommand

now it should work always (end of file, wrapped header, multi selection)
Merge pull request #5 from vovkkk/patch-4

adds class SmartNewLineCommand
Merge pull request #20 from alehandrof/nicer-folding

Nicer folding
fixes class SmartNewLineCommand

ctrl+enter should insert new line if headline isn't folded
Merge pull request #22 from vovkkk/patch-2

fixes class SmartNewLineCommand
Modify readme.md