Adds the following API functions.
- nvim_buf_set_mark(buf, name, line, col)
* Set marks in a buffer.
- nvim_buf_del_mark(buf, name)
* Delete a mark that belongs to buffer.
- nvim_del_mark(name)
* Delete a global mark.
- nvim_get_mark(name)
* Get a global mark.
Tests:
- Adds test to all the new api functions, and adds more for the existing
nvim_buf_get_mark.
* Tests include failure cases.
Documentation:
- Adds documentation for all the new functions, and improves the
existing fucntion docs.
49fdc62114
