Change behavior of 'o' when on a heading

[GuiltyDolphin]

Currently, the behavior of o when on a heading is to call evil-open-below, which looks like the following:

Before:

* foo<cursor>

After pressing o:

* foo
  <cursor>

Which is useful if you want to have properties for the heading:

* foo
  :PROPERTIES:
  ...
  :END:

But it may be useful to provide an option to change the behavior to do the following (after pressing o):

* foo

<cursor>

Another option might be to have o automatically insert the :PROPERTIES: boilerplate, and delete it if RET is pressed again without editing the properties. It might not make sense to handle this if snippets are being used (likely dealing with the boilerplate already).

[GuiltyDolphin]

Note that as of v0.4.0, using o/O with a prefix argument inserts a sibling heading after/before the current heading.