Make the demo easier to run

[simon-smart88]

You could make it easier to run your demo by moving the files to inst/demo which gets copied to your user's machine when they install. Then you can always find the files using list.files(system.file("demo", package="VariantSurvival"), full.names = TRUE)

[Krannich479]

Thanks for using VariantSurvival! Since version 0.2.0 a simple
VariantSurvival::VariantSurvival(demo=TRUE)
after the installation should automatically load the testdata into the dashboard.
We'll clean up the README soon!