Delimited text substring capability

This request adds substring functionality to the delimited  text plugin.  That you can define a subset string in the url, and update via the UI.

The main issue with this is that the URI opens an SQL expression dialog, whereas this uses the standard QgsExpression to select.  However the functionality is still all available - just the query builder doesn't quite match.  This will definitely go against inclusion in 2.0 (though in fact no different to existing issues with SQL variants in query builder)

Maybe wishful thinking for 2.0, but very useful capability and makes delimited text more compatible with other providers.