Add file_mode to set file permissions

[macladson]

Issue Addressed

#5

Proposed Changes

Adds a file_mode method to the builder to set file permissions in octal format. For example to set restrictive permissions:

.file_mode(0o600)

This also sets compressed files to these permissions as well.

This PR also contains a small architectural change (using self more liberally) which made a few things easier for me.

Considerations

Note that technically any u32 value will be accepted, even if it would break file permissions. For example .file_mode(600) is not equivalent to `.file_mode(0o600). It is highly recommended to use octal format otherwise you might get unexpected file permissions. There are a few validation techniques we could use, but none of them felt particularly clean. One option we could consider is an enum like this:

enum FileMode {
    Permissive, // Equivalent to 0o666
    Default, // Equivalent to 0o644
    Restrictive, // Equivalent to 0o600
    ...
    Custom(u32), // Users set their own mode.
}

Then for the non-Custom variants we could ensure the mode is set to octal, which then would only expose the footgun to users who opt in to a custom mode.

However this adds some bloat, and I appreciate the logroller is very lightweight, so we might just deem this footgun acceptable.