Flag extension functions

[hadley]

e.g. dplyr::intersects() makes base::intersect() generic so it shouldn't ever cause problems.

[hadley]

Latest syntax proposal:

conflict_superset("use_test", "usethis", "devtools")

As suggested in #8, conflicted will assume that any function that overrides a base function follows the superset principle, unless it has attribute conflicted_superset equal to FALSE.

[hadley]

conflict_superset() appears to be identical to conflict_prefer() so there's no need to implement.

dplyr::filter() and dplyr::lag() are the only functions that I know of that violate the superset principle, so I've hard coded them for now, instead of making extensible. We can reconsider in the future if other functions are discovered.