Add new customizable variable TeX-insert-braces-alist.

* tex.el (TeX-insert-braces-alist): New customizable variable.
(TeX-insert-braces): Mention it in doc-string.
(TeX-parse-macro): Use `TeX-insert-braces-alist'.

* style/booktabs.el: Update copyright years.
(LaTeX-booktabs-arg-paren): Let-bind `TeX-arg-opening-brace' and
`TeX-arg-closing-brace' instead of `<' and `>'.
("booktabs"): Add `toprule', `midrule', and `bottomrule' macros to
`TeX-insert-braces-alist'.
("booktabs"): Add a dummy `ignore' in `cmidrule' macro in order to
reset `last-optional-rejected' to nil.

* doc/auctex.texi (Completion): Document
`TeX-insert-braces-alist'.

* doc/changes.texi: Mention `TeX-insert-braces-alist'.
Add new possible value for `TeX-insert-macro-default-style'.

* tex.el (TeX-insert-macro-default-style): Add new possible value
`show-all-optional-args' and update doc-string accordingly.
(TeX-parse-arguments): Use it.

* doc/auctex.texi (Completion): Document `show-all-optional-args'.

* doc/changes.texi: Mention `show-all-optional-args'.