Allow global mutant operator config

[MattWindsor91]

Building on #126, another pattern is that many of the mutant operators work across machines; specifying them individually per machine is repetitive and error-prone. As such, it would be nice to:

• have a global configuration of mutant ranges, using the same sort of syntax as present (and likely stored directly in the plan's root);
• allow range specifications as follows:

[[mutation.auto.ranges]]
  operator = "LT"
  start    = 108
  end      = 111
# ...
[[machines.foo.mutation.auto.ranges]]
  include = "LT"

This would just expand to the global definition for LT.