Support setting configurables for Tools

[heralden]

An example of this is the jbrowse url: https://github.com/intermine/bluegenes-jbrowse-tool/blob/e911fb3a3c6e107252d2c746c6f662822cb3ae7e/config.json#L21

There are probably other examples of configurables user may want to set without having to fork the visualization tool.

This would be editable on the BG tool store page.