add --untrimmed-paired-output to changelog
rename diff() to files_equal()
tmp
Adapter sequences can now be read from a FASTA file.

For example, write `-a file:adapters.fasta` to read 3' adapters from
`adapters.fasta`. This works also for `-b` and `-g`.

To support this, the function gather_adapters() was factored out from
main().