Add a way to pass along command-line arguments with --exec

[nevinera]

qq -Xrspec is very useful, but I'd like to be able to supply extra flags to rspec, so I can write aliases that, for example, run with -fd, or filter the target specs with -e.

A lot of unix tools accept -- as meaning "pass along everything past this flag to the underlying tool - that's not useful in the normal qq use-case, but could work perfectly for qq -X.. is that too complex? How hard is it to document usefully?

Would we be better served by a general-purpose "pass this string as additional arguments to that tool" system, which could used through the cli or the config file? That would give us quite a bit more, but would require the flags to be quoted.. and currently we have no system for accepting non-boolean arguments per-tool on the cli (file-filter is config-file only). Perhaps we can fix that first, implementing a general-purpose way to supply per-tool argument-taking options, and then use it for this?

It might be ideal to support both approaches, to simplify the quoting issues, but only use the post--- arguments in the -X case?