Add support for oc:regexp-posix extension

As noted in the long discussion in https://github.com/openconfig/public/issues/44 , OpenConfig now tags its modules with a specific extension:

https://github.com/openconfig/public/blob/master/release/models/openconfig-extensions.yang#L106

like this:

https://github.com/openconfig/public/blob/master/release/models/stp/openconfig-spanning-tree-types.yang#L72

This allows us to modify our parser logic to interpret pattern statement arguments as POSIX patterns rather than XSD patterns, thus fixing the $/^ issue (and others).