Allow manual naming of mitochondrial genes

[wmacnair]

"It would be great for the user to have a parameter or something to add the names of the mitochondrial genes to make_qc_dt manually. She had mouse samples and just got a warning that no human mitochondrial genes were detected in the first step."

@mluetge in case you want add anything

[wmacnair]

My own thoughts:

I'm still undecided on whether I should put effort into a function for calculating QC metrics quickly, as scater already does this. I'll consider it if enough people find the scater function slow for v large datasets, or there's some other reason.

[wmacnair]

Closing as I'm going to steal the QC metric functionality from scater