Add operators start-with and end-with

[mdbenito]

This PR adds support for startWith and endWith.

The implementation allows for an arbitrary number of values to be passed as args, as in

(->> (rx/from [3 4])
        (rx/start-with 1 2))

to produce [1 2 3 4]

I thought this to be more convenient than always taking an array of values, since the typical use case is (start-with val ob) with just one value.