use local headers for post attributes in latex source blocks

[matthuszagh]

The current use of

attr_wrap(orgwidth="", htmlwidth="", name="", caption="", data=*this*)

is a bit cumbersome, even if it can be set as a default header. The reason is that whenever you need to override one of the arguments, you must override the whole thing. It should be possible to compose the same effect with my upstream org-mode patches and "local" header arguments. Local header arguments are a new feature I'm using where I prefix the header with an underscore. For instance, :_class. Org-mode passes these through and they are accessible by my default header argument functions. Because we can compose header arguments out of local header arguments we can compose attr_wrap this way.

An additional relevant question is whether we can concatenate the local headers. For instance, it would be useful to be able to do something like

#+header: :_caption Start of some caption
#+header: :_caption that continues on the next line.