ENH: resample().interpolate()
From https://github.com/pydata/pandas/pull/12449#issuecomment-211899951

When downsampling on a Resampler object, you now have different `fillna` methods to fill the NaNs (or `asfreq` for a plain reindex like operation without NaN filling). I can possibly make sense to also have `interpolate` available to fill the missing values directly (instead of first calling `mean`/`asfreq`)

```
In [20]: df = pd.DataFrame(data=[1,3], index=[dt.timedelta(), dt.timedelta(minut
es=3)])

In [21]: df
Out[21]:
          0
00:00:00  1
00:03:00  3

In [22]: df.resample('1T').mean().interpolate('linear')
Out[22]:
                 0
00:00:00  1.000000
00:01:00  1.666667
00:02:00  2.333333
00:03:00  3.000000
```

Should `df.resample('1T').interpolate('linear')` give the same result?
