please add option to force overwrite .fai index during, or automatically delete fai file after barrnap run

[jvollme]

Barrnap creates an index file of the query sequence before searching for rRNAs. if this inex already exists, it skips the creation of this index. This can lead to problems, if the query file has been edited since the last barrnap-run. This is especially a problem, when integrating that tool into custom pipelines, where one would avoid deleting problematic files simply based on their names as such files may exist for other reasons also. Tehrefore it would be best if each tool could just "clean up behind itself"...