[vim] Implement aB, iB, ab and ib text objects with an operator.

This code implements and tests ab/ib/aB/iB with an operator as indicated below. *Todo all the remaining x's.

      -----------------------
      | text-object | o | v |
      -----------------------
      | aw          | y | x |
      | iw          | y | x |
      | aW          | y | x |
      | iW          | y | x |
      | as          | x | x |
      | is          | x | x |
      | ap          | x | x |
      | ip          | x | x |
      | a] a[       | y | x |
      | i] i[       | y | x |
      | a) a( ab    | p | x |
      | i) i( ib    | p | x |
      | a> a<       | x | x |
      | i> i<       | x | x |
      | at          | x | x |
      | it          | x | x |
      | a} a{ aB    | p | x |
      | i} i{ iB    | p | x |
      | a" a' a`    | y | x |
      | i" i' i`    | y | x |
      -----------------------

  Legend:
    o: Operator such as d, y, c, etc.
    v: Visual mode (including blockwise & linewise visual mode)
    y: Implemented
    p: Newly implemented b & B.
    x: Not implemented