Add --bytes/-b flag to display byte counts instead of human-readable values

[nadesai]

This PR addresses #6, though very inelegantly.

• Creates a new flag --bytes that displays byte values instead of human-readable values for all output. (I put Google Translate-based guesses in for French and German translations, as I don't speak those languages.)
• Modifies the (unused) display_item function to work with any reference to T implementing Display; also adds a display_bytes flag. Updates all code locations where file or directory sizes are printed so that this function is used. (This introduces a check that size != FileSize::new(0) in all print statements, which does not match prior behavior e.g. at https://github.com/vmchale/tin-summer/pull/12/files#diff-9801ad6b6d07be3a4b68f9fd9e3b9832R273 or at https://github.com/vmchale/tin-summer/pull/12/files#diff-5899cfb17ab780a6ee6255135e9f06feR680. This would be easy to rectify if there is a concern.)
• Modifies the read_all, read_all_fast, display_tree fns, and the Walk struct, to add parameters or fields capturing whether or not data should be displayed as bytes.
• Modifies relevant invocations of all these functions to take the correct parameter for displaying the bytes value.

Some things still need to be addressed:

• No tests exist to validate behavior with -b.
• Length of byte value strings varies wildly, so during printing with byte values, not all file names or directory names start at the same column.

Please let me know what I can change in this PR to make it usable - this is my first time attempting to contribute to this project.