Change to using `:package' attribute on example lists
* lisp/ox-linguistics.el: find-enclosing-env -> org-linguistics-find-enclosing-pkg;
  - change code that looks for :environment to use :package instead
  - convert keywords to package names (e.g. "gb4e-exe" -> "gb4e")
* README.md: update docs to explain use of :package attribute
Add support for :environment on gb4e examples
* lisp/ox-linguistics.el: org-linguistics-gb4e-plain-list: allow user
  override of environment via :environment attribute
Prepare for supporting different item commands
* lisp/ox-linguistics.el: org-linguistics-PACKAGE-item-commands variables;
  docstring and comment additions
* lisp/ox-linguistics.el: use ATTR_LINGUISTICS instead of ATTR_LATEX
* README.md: update to reflect new ATTR_LINGUISTICS syntax
* README.md: fix quotation typo
* lisp/ox-linguistics.el: simplify org-linguistics-linguex-item
  (org-linguistics-linguex-item): use new(?) functions
  org-export-first-sibling-p and org-export-last-sibling-p, provided
  by Org's exporter, in place of inline functions
* lisp/ox-linguistics.el: remove dependency on cl
  (org-linguistics-gb4e-item): replace unnecessary (labels ...)
  with (flet ...)
* lisp/ox-linguistics.el: complete ATTR_LINGUISTICS conversion
  (org-linguistics-item): check :attr_linguistics, not :attr_latex for
    :package attribute
  (forgot to stage this in 0c90fe04)
* lisp/ox-linguistics.el: remove half-start on handling for other item
  commands

I've decided that what I've got is enough.  The user can already
override the default labeling scheme by providing a list tag; and
supporting the other commands doesn't gain us much, since these
commands seem to be pretty broken.

For example, gb4e's \exp command breaks label/ref.  Although it does
produce a "primed" example in the output PDF, it doesn't give you a
way to refer to that primed example; you can't say

\exp{foo}{\label{foo-prime}My primed example}

and then later \ref{foo-prime}.  The reference doesn't appear in the
output. (Argh!)

Producing the correct output for the primed example in this case can
already be done using the list tag mechanism (which exports to the
\exi command in gb4e), and so nothing is gained by providing support
for \exp as well.

The only case where it would actually be useful to support a different
underlying command is glosses, and I think that can be done in a
different way.
Merge branch 'package-command-split'
* README.md: document list tag mechanism for overriding default numbering