support trimming of paired end reads by interleave -> trim -> re-pair

This set of commits allows something like:


```shell

./seqtk interleave R1.fastq R2.fastq \
   | ./seqtk trimfq  - \
   | ./seqtk pairfq -  

```

So that the existing machinery can be used to trim paired-end fastq files. Or **any single-end trim command can be used in place of `seqtk trimfq`**

This can be checked by appending this to the end:

    | awk 'NR % 4 == 1 { print $1 }' | uniq -c | awk '$1 != 2'

which should give empty output when all reads are paired.



It would be nice of the cpy_kseq command was included in kseq.h