I have a pandas dataframe with column values in string format and a datetime index. I want to create a new column which will have a list of values of a column for last two days. Is it possible to achieve this using pandas?

original datafarme:

```
        date col1 col2
0 2018-07-08    a    b
1 2018-07-09    c    d
2 2018-07-10    e    f
3 2018-07-11    g    h
4 2018-07-12    i    j
5 2018-07-13    k    l
6 2018-07-14    m    n

```

Final dataframe:

```
        date col1 col2  col3
0 2018-07-08    a    b   NaN
1 2018-07-09    c    d   NaN
2 2018-07-10    e    f  b, d
3 2018-07-11    g    h  d, f
4 2018-07-12    i    j  f, h
5 2018-07-13    k    l  h, j
6 2018-07-14    m    n  j, l

```