Implement basic feature
Fix typo, implement first TODO
Add extra TODO, fix typos
Don't pursue cleanup after buffer kill anymore

Burying a "transient" buffer usually closes it together with the window
if it wasn't meant to reuse the current window anyways and is the ideal
cleanup behaviour.  In case this doesn't work, closing its window works,
too.  Killing it would introduce unforeseeable consequences and is
generally complicating matters, therefore I won't pursue making that
work anymore and will document it in a later commit.
Clarify comment
Move function to more sensible spot
Add documentation
Fix docstring quoting
Make alignment customizable
Introduce ratios
Fix up docstrings
Explicitly use splittable window

By going that way, a bug is avoided that made `helm-complete--in-region`
error out when only one window was visible and TAB hit in a `M-:` prompt
since it assumed the splitting size from the minibuffer which obviously
will not work for splitting a window.
Implement error handling for ratios
Conform to convention