Use last for length 2 vectors with ansi_collapse() when sep2 not used

[rundel]

This addresses #681 - rather than always using the sep2 separator the new code does the following for the vector length 2 case:

• If last is non-default (i.e. not ", and") and sep2 is default (i.e. " and ") use last rather than sep2 when collapsing

• Otherwise use sep2 when collapsing

[gaborcsardi]

Thank you! In the end I chose a different approach, but your PR has very helpful, still.