BUG: loffset not applied when using resample with agg() (GH13218)
- [x] closes #13218 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13861?src=pr) is 85.29% (diff: 100%)

> Merging [#13861](https://codecov.io/gh/pydata/pandas/pull/13861?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13861   diff @@
==========================================
  Files           139        139          
  Lines         50164      50168     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42788      42791     +3   
- Misses         7376       7377     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [257ac88...dbb054f](https://codecov.io/gh/pydata/pandas/compare/257ac884d61a74990d1cb4d72c48b1c9003298d5...dbb054f9b787dbbe09ffa60424e19d1b1a0d973f?src=pr)
 in any event, pls add the test for timedelta index, but assert it fails (and put a TODO for the xref issue #7687 )
 @jreback I'm not quite sure what you mean by put a TODO, could you please clarify? 

Anyways, doing the tests was kind of tricky, so please lmk if I went in the right direction.
 what I mean is can leave the timedelta fixes for another PR to fix
but u can out the tests in for them (and since they will fail u can just assert that they fail and add a comment, a todo for fixing later)
 @jreback for future reference, when I update my PRs should I @ someone with write access to let them know?  Or should I just wait, and check on them intermittently, rebasing when they get out of date?

thanks
 you can ping me if u want me to look

I added some comments
 @jreback split the tests into separate test classes
 @jreback I think I explained the use of `arg` sufficiently. 

I also didn't realize that `loffset` was broken when passing in a list to agg, e.g. `df.resample('2d', loffset='2h').agg(['mean']).`, so I added a fix and put in the corresponding tests.
 @jreback when you have time can you take a look at this, thanks
 @jreback I've tried, but can't think of any more alternative solutions. I went back to what I had originally, with the logic in the `aggregate()` functions. 

if this is insufficient, please lmk if you want me to close the PR
 well, then reopen if you have a better way.
 @jreback I force pushed but I don't have the option to reopen. I don't wanna create a dupe, so is there any way you can reopen this? 
