Add templates

[jonniedie]

Since startup files don't work with the reproducibility model of Pluto, it would be nice to instead have a way to achieve the same thing with template files. So you could have a notebook with some statements at the top like setting environment variables (say, for plot backends), or some usings that you're always going to want imported in your notebooks. You'd then call Pluto.run("<filename>") or something to open up a notebook with those cells pre-populated at the top.

[fonsp]

Maybe this can be achieved by allowing the user to customize the list of sample notebooks. Maybe everything inside ~/.julia/pluto_sample_notebooks/ can be added

[jonniedie]

I think that would be a great idea.