Adding --source option to load_mitochondria.pl script

## load_mitochondria.pl 

### New features
- added --source option to the load_mitochondria.pl script 
- added MIT_SOURCE_NAME  parameter for default source name (ensembl) to config file 
- added some example command line on how to use the script 

### Bugfix
- fixed bug which prevented users to use the --logic_name option to set the logic name of the MT genes directly