Extract multiple mito contigs

[Euphrasiologist]

The time has come.

• gfatk extract-chloro should only extract one subgraph as we expect a consistent structure.
• gfatk extract-mito should allow for multiple contigs being extracted. These contigs are usually single segments, and circular. So we can restrict our extracting criteria to these at the moment, unless there is good evidence otherwise.

[Euphrasiologist]

This has been fixed in the last few commits, forgot to reference this issue. Ooops.