BEAMER export: Allow overlay options for column environments

BEAMER export: Allow overlay options for column environments

* lisp/org-beamer.el (org-beamer-sectioning): Allow overlay arguments for
the column as well.
* doc/org.texi (Beamer class export): Document that also overlay arguments
can be passed to the column environment.

Eric Fraga writes:

> I am trying to create a beamer slide which has two columns.  The second
> column should only appear after a while (the 6th uncovering operation).
> In latex, I would do:
>
> : \begin{column}<6->{0.4\textwidth}
>
> say.  In org, I would expect to be able to get this latex code generated
> by the following:
>
> --8<---------------cut here---------------start------------->8---
> ***** column heading			      :BMCOL:B_block:
>      :PROPERTIES:
>      :BEAMER_col: 0.4
>      :BEAMER_envargs: c<6->
>      :BEAMER_extra:
>      :BEAMER_env: block
>      :END:
> --8<---------------cut here---------------end--------------->8---
>
> according to the info documentation (Beamer class export).
>
> However, this does not work: the "c<6->" is placed verbatim in
> the \begin{block} that comes after the \begin{column}.  Furthermore, if
> I ask for the heading to be ignored (instead of defining a block), the
> envargs are lost completely!