allow alternative filter for 'ecoli-position'

[epruesse]

When working with Eukaryotes, the greater length of the 18S makes it sensible to use an 18S to compute reference positions. Typically, Saccharomyces is used.

It's unlikely that there will ever be more than two alternatives, including "one for the ", three will suffice. Quick and easy switching (I doubt we can show them at the same time) is more important.