Allow merging of cassette placeholder overrides

[ekohl]

This changes the cassette placeholders parameter to always use the default cassette options and then replace any matching keys. To do so it first replaces the storage of placeholders from a list of dicts to a dict.

It maintains API compatibility so users don't have to modify their tests, but because of the merging it could invalidate existing cassettes.

Another issue is that Cassette.default_cassette_options is a class variable meaning you can't remove any global config keys. This could be solved by moving the defaults to the configuration and passing it in the constructor.

Closes GH-97

[sigmavirus24]

I rebased and merged this. Thanks @ekohl

[ekohl]

Thanks!