fix: functions were modifying input options object

[RoystonS]

The externally-facing looks-same functions were mutating any options object passed into them.

This was problematic because if you wanted to reuse an options object for multiple calls, it would cause failure. For example, if you had an options object that simply had {strict: true}, then during the first call, looks-same would add a threshold property to that, and then complain about the presence of both strict and threshold on the second call.

This PR adds tests to ensure that the options objects are not mutated, and applies two types of fix:

1. _.defaults mutates its first parameter, so add an extra empty {} as the initial parameter. This way, we accumulate the results in a new object. (It would be even simpler to use an object spread but I don't see any in this codebase and it might cause compatibility issues.)
2. Don't assign directly to properties on opts, but combine that defaulting with ._defaults calls.

This fixes issue #41