improved gitignores

Simplified and added new gitignores for the following directories:

- `magmi/conf` - instead of ignoring a long blacklist, we're changing the strategy to blacklist everything in this directory and whitelist the default .ini file as we cannot guess what profiles can be generated here.
- `magmi/state`, - about the same concept as above, but is defined in the project root instead.
- `magmi/plugins/base/datasources/csv/` - ignore the generated downloads folder (if using remote CSV source in a profile)