Add options to createWriter() to deal with atomic writes.

[mkruisselbrink]

This attempts to solve #37 by making it explicit that changes made to files
might not be reflected on disk until flush() or close() is called on the
writer.

This also introduces a "atomic" mode for FileSystemWriter where any changes
are written to a temporary file, and only on flush or close is that file
moved on top of the destination file. This should address #48.

Still to be decided if we actually want to implement the non-atomic mode at
first, as the semantics there aren't very clear. I.e. we might want to start with only
having an atomic mode.

[oyiptong]

Instead of a flag, how about an enum?

fopen provides the following options closest to what we want to achieve:

• w for truncate mode
• r+ for read/write, with the cursor at the beginning of the file

I like the idea of update to be the opposite of truncate.

If we want to contrast with fopen so there's no confusion, we can use slightly different enums:

enum FileSystemWriterMode {"truncate", "update"};

dictionary FileSystemCreateWriterOptions {
  FileSystemWriterMode mode = "truncate";
  boolean atomic = true;
};

[mkruisselbrink]

I went with victor's suggested names for now.

[pwnall]

FileSystemCreateWriterOptions is a great idea! Let's get that in.

I'd prefer that we discuss flush() separately.