trimfq to drop seq under minlen and output cutpoints

[a1an77]

Hi, I needed to change the behaviour of the trimming function to drop reads shorter than minlen after trimming instead of providing low quality bases and I added an option to output the cut points (-1 if discarded) in a separate file. If you like the changes to be included I can open a pull request. Also added a Q option to set the threshold with a Phred score

[tseemann]

I think you can just seq trimfq ... | seqtk seq -L minlen ...

Might also be able to use seqtk seq -q PHRED .... to do the quality trimming.