What is this plant that grows like a weed in my yard (I live in Massachusetts). It has very small 5-pointed flowers which are blooming right now (2nd week in June). The flowers are purple. The plant grows somewhat like a vine and has a tendril, but does not climb very high and it seems to like the ground. The flowers are quite small, about the size of an eraser on a pencil, or slightly larger.
The leaves are simple, opposite and spade-shaped.