Configurable strings; optional group headings

This takes in Synchro's changes but then changes it a bit more, changing the format of the insertable strings to be a list of dicts so that we set the name, value, and voice label explicitly.  In this way only value is required for insertable strings, but if just a name is provided it is used as a heading for the strings that follow.

Most importantly, of course, the strings are now taken from a configuration setting, so they can be set without editing the plugin.

All this is documented in the much-expanded README now.

Looking forward to seeing this at http://ckeditor.com/addon/strinsert !  :-)