Implement pretty fasta

The following commits implements a new parameters to fasta to make the output more appealing to the way. Using something like

    -width => 100,
    -block => 10,

will now make a nicely formatted output with a space each 10 bp/aa. This allows for easy identification of any position.

I'm not sure about the necessity of making WIDTH a package variable so I let it be. I think it should be removed unless it's meant to be changed by the user directly. But it seems to be undocumented so my guess is that is not wanted.

Carnë