Add option to output file size and date/times in a machine readable format

[athena-metis]

The usual command flag would be -h to make it human readable but as this is already the default behaviour implementing it in this way would require a change to existing functionality that others may rely on. This change is to add a slightly less conventional switch to allow the output to be more friendly to integrating with scripts.

The change allows for a -m / --machine flag for rev/search/ls commands which will output the file size in bytes and the date in a standard format rather than "X hours ago" etc. This allows for use in scripts where you want to compare a file size or check the modified date (separate PR for maintaining this when uploading via the put command)

The -m / --machine switch implies -l / --long as the standard ls output doesn't include any humanised information anyway.

Existing long output

$ dbxcli ls -l
Revision              Size  Last modified Path
5a3f4f259f9d3554dee35 548 B 2 hours ago   /LICENSE

$ dbxcli ls -m
Revision              Size Last modified                 Path
5a3f4f259f9d3554dee35 548  2020-04-23 13:08:36 +0000 UTC /LICENSE

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