Option to control parallel file processing

[andrews05]

Processing multiple files in parallel can bring some nice speed improvements but can also cause problems:

• Higher memory usage (memory allocation of x bytes failed #683)
• Confusion with verbose output (Race condition in verbose output #572)
• Non-deterministic order of files processed ([Question] The order of files that gets processed changes randomly? #590)

We need a switch to control whether parallel processing should be enabled or not.
The question is whether this should on by default and require (e.g.) --no-parallel-files to disable it, or off by default and require --parallel-files to enable it.
Keen to hear thoughts on this.

[TPS]

I vote for parallel by default, but w/ gracefully scaled auto-fallback as requested when there aren't enough resources for fully-parallel processing.