Support for local files

[mf-rug]

As far as I can, the molecule source is currently required to be a www URL / database id. It would add a lot of value to this promising package if it also supported local file input.
The code comments mention this

#' The `file` parameter is supported by the react library, but it should not
#' be used in Shiny.```

But I'm unsure why this is.

Note that I managed to circumvent the issue by placing the local in file in the app's www/ folder and then:

pdb_url <- paste0(session$clientData$url_protocol, "//",
                  session$clientData$url_hostname, ":",
                  session$clientData$url_port, "/",
                  pdb_filename)

Molstar(url = pdb_url, useInterface = TRUE, showControls = TRUE)

But native support would be great. Thanks!