Add sequence name parameters to visualization functions

[fedarko]

The default axis labels are useful, but they just say "s1" and "s2" which is not usually helpful. You can fix this by calling set_xlabel()/etc. directly, but then you have to manually add back the length info and the arrow indicating direction.

It would be nice to be able to specify just sequence name(s) to the visualization functions, which replace the s1 / s2 things without replacing the length info / arrow.