Delimited text substring capability - also bug #7742

This request adds substring functionality to the delimited  text plugin.  That is you can define a subset string in the url, and update via the UI.

This also means that delimited text data can be joined to other layers (which requires  subset string support).  Note that at the moment the implementation is not well optimised for this use. However for small tables it can be very convenient.  

The main issue with this is implementation that the vector layer properties dialog 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.  

Maybe wishful thinking for 2.0, but very useful capability and makes delimited text more compatible with other providers.  Also very safe in terms of impact - doesn't affect API and scope is completely contained within delimited text provider (apart from additional documentation comments in vector data provider header file)

Also this has been raised as a regression bug http://hub.qgis.org/issues/7742.   I'm not sure if technically it is a regression (ie I'm not sure if it was ever working).

The user manual http://docs.qgis.org/1.8/html/en/docs/user_manual/working_with_vector/vector_properties.html#joins-tab states joins to delimited text providers is supported, but it doesn't work for me in 1.8?