support markdown list markers

Now possible to mark as done/undone items in markdown list:

- [ ] minus
+ [ ] plus
* [ ] multiply
fix cursor position on lines with leading whitespaces
respect list marker on create item above or below

While create a new item above or below uses the list marker used in current
line. Indentation level remains the same.
update sections in code
Merge pull request #6 from vitalk/markdown-compatibility

Support markdown list markers. Closes #5.