feat(aria/combobox): allow custom popup triggering logic

[mmalerba]

Currently I don't see a way to disable the default popup triggering logic for ngComboboxInput. It always pops up when I type in the input, however https://www.w3.org/WAI/ARIA/apg/patterns/combobox/ seems to indicate that the behavior should not be quite so persecriptive:

Some possible conditions that trigger expansion include:

• It is displayed when the Down Arrow key is pressed or the Open button is activated. Optionally, if the combobox is editable and contains a string that does not match an allowed value, expansion does not occur.
• It is displayed when the combobox receives focus even if the combobox is editable and empty.
• If the combobox is editable, the popup is displayed only if a certain number of characters are typed in the combobox and those characters match some portion of one of the suggested values.

I think the third example condition in particular is not captured well by the current design. I also think this is a common use case for things like editors with chips (e.g. like in the Github comment UI, when I type in a comment "fix my issue @wagnermaciel 😛" the popup triggers only when I type @ and then again when I type :)