text: Title: Conditional join
  Body: # Brief Description
  
  <!-- Please provide a brief description of what you'd like to propose. -->
  
  I would like to propose conditional join, also known as non-equi joins, which is obtainable in SQL and R's datatable.
  
  # Example API
  
  ```python
  df1 = pd.DataFrame({'col_a': [1,2,3], 'col_b': ["A", "B", "C"]})
  	col_a	col_b
  0	1	A
  1	2	B
  2	3	C
  
  df2 = pd.DataFrame({'col_a': [0, 2, 3], 'col_c': ["Z", "X", "Y"]})
       col_a	col_c
  0	0	Z
  1	2	X
  2	3	Y
  ```
  
  Please modify the example API below to illustrate your proposed API, and then delete this sentence.
  
  ```python
  # the join can be >, < or !=. I guess as we get a stable function, we can add others, and combine with equi-joins
  df1.conditional_join(df2, left_column = "col_a", right_column = "col_a", join_operator = ">")
      col_a   col_b   col_c
  0	1	A	Z
  1	2	B	Z
  2	3	C	Z
  3	3	C	X
  ```
  