Add globstar (wildcard deep matching) support

[gustavohenke]

In a nutshell, any field paths with ** will now be considered a wildcard that can deeply match fields.

For example:

Input path	Input object	Selected fields
**	{ a: { foo: 1, bar: 2 }, b: { c: { d: { e: 3 } } } }	a.foo, a.bar, b.c.d.e
**.foo	{ a: { foo: 1, bar: 2 }, b: { c: { foo: 3 } }, foo: 4 }	a.foo, b.c.foo, foo

Works with both check() + variants, and checkExact().

Fixes #1137

[coveralls]

Coverage: ?%. Remained the same when pulling 4d898a7 on v7-globstar into 6c6ea73 on v7-features.

[coveralls]

Coverage: 100.0%. Remained the same when pulling 4d898a7 on v7-globstar into 6c6ea73 on v7-features.