Make opened modules read-only by default

[nfischer]

To me, it makes a lot of sense to make some modules open in read-only when opened by gf or :Nedit and the like. It doesn't make much sense to be able to edit the fs module, which is a standard module.

On the other hand, it makes more sense to be allowed to edit locally downloaded modules. The distinction of course can be easily made, since standard modules such as fs must be curled, while local modules are found locally. So the same logic could be used to set readonly and set nomodifiable.

[nfischer]

This is resolved by #31.

Core modules are opened in readonly mode, and local files are opened with standard permissions.

A possible extension to this would be to open dependencies (things inside node_modules/ and the equivalent for global packages) in readonly as well.

[moll]

Surprising that Netrw itself doesn't set them readonly... I can understand not doing so for protocols that have a standard for uploading files, but for HTTP...