Added ability to passThrough() if its set at beginning of all handlers

[meetromb]

Example of a problem:

const mock = new MockAdapter(instance)
mock.onAny().passThrough()

mock
    .onGet('/foo')
    .reply(200, 'bar')

mock
    .onGet('/bar')
    .reply(200, 'foo')

In this case every request will be forwarded to origin instance.

According to the documentation you need to do this:

const mock = new MockAdapter(instance)

mock
    .onGet('/foo')
    .reply(200, 'bar')

mock
    .onGet('/bar')
    .reply(200, 'foo')

mock.onAny().passThrough()

But this is inconvenient, because the code can be in different files and run out of order. This PR fixes this.

[ctimmerm]

This would be a breaking change and make the API inconsistent, I think it would be better if there was a separate way to define what should be done when no matching handler is found. Maybe this could be an option that is provided when the mock adapter is created, what do you think?

[ostapenko-me]

This would be a breaking change and make the API inconsistent, I think it would be better if there was a separate way to define what should be done when no matching handler is found. Maybe this could be an option that is provided when the mock adapter is created, what do you think?

I corrected this remark in the request #237. Can you check it out?

[ctimmerm]

Let's continue this in #237.