add withContext option

[searls]

Proposal: allow test double stubbing/verification configuration to specify the value of this that's bound when the test double function is called.

td.when(myTd(), {withContext: window})).thenReturn(5)

myTd() // undefined
myTd.bind(window)() // 5

And for verify:

myTd()
td.verify(myTd(), {withContext: window})) // blows up

myTd.bind(window)()
td.verify(myTd(), {withContext: window})) // passes

Also, argument matchers should be supported:

myTd.bind(new Promise())()
td.verify(myTd(), {withContext: td.matchers.argThat(arg => arg instanceof Promise)})) // passes

Thoughts?