Provide a non-threadsafe mechanism to override registered values

[billdueber]

Uses a simple push/pop with the current context (registry, cache, etc.) to override withing the push/pop cycle, or use container.override { ... } to automatically push/pop at the beginning and end of the block.

Did I say "non-threadsafe?" OVERRIDES ARE NOT THREADSAFE!

[billdueber]

Geez. Realized I didn't commit a couple files. Fixed.

[aelkiss]

This looks pretty nice; I want to give it a try with some of my projects where I override things in tests to make sure that it works as expected.

[moseshll]

Spent a bit of time with this branch and hathitrust_catalog_indexer and overall I'm liking it. There is some behavior in this branch that I believe violates the principle of least astonishment, however. To wit:

irb(main):002:0> c = Canister.new                                                    
irb(main):003:0> c.push_context!                                               
irb(main):004:0> c.register(:a) { "b" }                                                                         
irb(main):005:0> c.pop_context!                                                                          
irb(main):006:0> c[:a]
=> "b"

Expected behavior would, for me, be a raised exception since the registration was made in a pushed context.
@context.dup defaults to a shallow copy of the existing context, so the @registry hash is shared between all contexts. I tried this and was satisfied with the behavior:

def dup
  self.class.new(registry: @registry.dup)
end

(I should also note that doing c[:some_key_i_havent_registered_yet] leaves junk on the @stack but that's an issue for another day.)

[moseshll]

See main comment. (Figured I might as well turn this into a real review.)

Probable Scope Creep Initiate!
I also find the cited behavior of e.g., c[:some_key_i_havent_registered_yet] (raising NoMethodError and leaving junk on the stack) somewhat jarring. Surely we can act more like Hash (which is ultimately how I conceptualize this class) or at least return a more appropriate exception class.