Create more minimal plugin template

[cmdcolin]

It may be useful to make a couple different jbrowse-plugin-template's also to try to reflect diversity that some users might have...some users may want a more minimal template than this one for example

[garrettjstevens]

I'm finding myself wanting this with Apollo. Apollo plugins are JBrowse plugins that target Apollo-specific extension points, and there are a lot of things in this plugin template that Apollo plugin developers might never need.