Add method to open tracks without default_session?

[cmdcolin]

Currently the "default_session helper" is used for opening tracks by default. however, this doesn't necessarily translate to the config.json scenario cleanly. could add extra helpers, or figure out a good workflow.

recall: in our @jbrowse/react-linear-genome-view storybook, we actually chose to not use defaultSession, and to imperatively call showTrack on the created view model after initialization.