add array bracket notation support

[mreinstein]

this works now:

const obj = { a: [ { b: 'cool' } ]};
const val = dotProp.get(obj, 'a[0].b'); // val === 'cool'

[sindresorhus]

Try to come up with some more assertions. For example, a[0].nonexistant with and without default value.

[sindresorhus]

Also see #71

[sindresorhus]

This will need docs and more tests (for each method).

[mreinstein]

Also see #71

Yeah, I saw that when looking through back issues to see if this was brought up anywhere. The title of 71 matches what this PR does, but the content of the issue seems somewhat unrelated (claims to backwards incompatibility breaks and other stuff)

This will need docs and more tests (for each method).

I didn't do the additional tests in the PR because all of the public methods (get, set, has) use the same underlying getPathSegments to resolve the path, so they seemed redundant. That said I'm happy to add them if you really feel those tests aren't redundant. Can you give me a sense of the depth of testing you want for each public method?

[mreinstein]

I have significantly underestimated the complexity of making this change. :(