[parser] postfix sub-slicing syntax

Issue #15's second code snippet is also a change in 0.12.  From the release notes:
* The syntax for matching of sub-slices has been changed to use a
       postfix `..` instead of prefix (.e.g. `[a, b, c..]`), for
       consistency with other uses of `..` and to future-proof
       potential additional uses of the syntax.

This PR fixes the parser to handle this syntax.