[rst mode] Rewrote from scratch

The old mode was not supporting many rST constructs, or not in
sufficient detail. Features/changes:

 * no `backUp` ops to keep things fast;
 * kept `python` inner mode integration;
 * added `latex` inner mode;
 * eliminated `verbatim` configuration parameter;
 * eliminated non-standard tokens: the old mode was producing `footnote`
   and `citation` tokens, which were replaced with `quote`;
 * many more details: visually compare highlighting between old and new
   to get an impression;
 * MIT license (copied and adapted form `python` mode);
Allow modes to specify their own flattenSpans option

And use the editor-wide option when no value is found on the mode.

Issue #1281