changed raise to warning for no sequence found

Originally, if no sequence was found, get_genome raised a
NoMatchingSequence found warning, which turns out to be a bit of a
hack.  The module now issues a warning through warnings.warn.
add interface

Functions were renamed to better reflect what they are doing, and
hidden behind an interface: get_taxon_name(…)
Created a flexible CLI generator

This script contains a class for the creation of simple, but flexible,
CLIs.  It works by generating a “call” function on the fly, which is
named the name of the program.  In essence, it creates a method for
calling that function.

Also, command line options can be added, and toggled at will.  Thus, a
single CLI object can handle any number of programs, each with unique
command line options.