Throw exceptions if key path does not exist

[colinodell]

In order to avoid constant null-checking I would like the ability for this library to throw exceptions when trying to access something that does not exist. Specifically, I would like to change these behaviors:

// Fetching a non-existent key with no default given
$data->get('foo.bar'); // returns null

// Fetching a non-existent key with `null` set as the default
$data->get('foo.bar', null); // returns null

// Fetching a non-existent key with a non-`null` default
$data->get('foo.bar', 42); // returns 42

To this:

// Fetching a non-existent key with no default given
$data->get('foo.bar'); // CHANGE: would throw an exception instead of returning null

// Fetching a non-existent key with `null` set as the default
$data->get('foo.bar', null); // returns null

// Fetching a non-existent key with a non-`null` default
$data->get('foo.bar', 42); // returns 42

Obviously this will be a BC break and therefore require a bump in the major version. Anyone wanting the old functionality could set null as the second argument.

ArrayAccess

One outstanding question is what the behavior of offsetGet() should be - calling this indirectly like $data['foo.bar'] does not provide the ability to define a default value. I have no preference on whether this should return null or throw an exception.