Improved documentation of CLI "dialplan add extension" command.

Improved documentation of CLI "dialplan add extension" command.

* Documented dialplan add extension <exten>,<priority>,<app(<app-data>)>
format.

* Allow acceptance of command without the app-data value.  There are many
applications that do no need any parameters so it is silly to require that
field for all commands.

* Fixed a couple ast_malloc/ast_free mismatches with ast_add_extension2()
calls.

(closes issue ASTERISK-19222)
Reported by: Andrey Solovyev
Tested by: rmudgett

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@354216 65c4cc65-6c06-0410-ace0-fbb531ad65f3