Append or not append a hash string to the name of a file when ```to_xml_string()``` is called

[barikata1984]

To deal with issue #483 without editing the source code locally, allow users to select to add a hash string after the name of a file when it is registered to VFS by specifying an optional boolean argument filename_with_hash. Sample usage can be found here.

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[yuvaltassa]

Thanks for this!

1. Can you please do the CLA thing?
2. Do I understand correctly that you're changing the default behavior?

[barikata1984]

@yuvaltassa

1. CLA completed.
2. No, the default behaviour won't change. The default value of filename_with_hash is set at True. This also applies to intermediate methods, e.g. to_xml(), _attributes_to_xml(), and so on, up to get_vfs_filename(). As a result, if a user does not provide filename_with_hash or does explicitly set filename_with_hash=True, a hash string is appended to each file name. Below is just a quick demonstration that calling to_xml_string() without setting filename_with_hash=False registers a file with a hash string appended to the file name:

[yuvaltassa]

Hopefully @saran-t will get to this soon.

[saran-t]

Merged in b2644081ae31c2adbe5a8319f0b9695e3ef9f8e1