Allow fastqscreen inputs to just be a fasta

### Description of feature

Kinda similar to #21.

I wish I could do: 

```
name,dir,basename,aligner
Ecoli,s3://ngi-igenomes/igenomes/Escherichia_coli_K_12_MG1655/NCBI/2001-10-15/Sequence/genome.fa,genome,bowtie2
```

And the workflow would build the reference for me on the fly as well.