Add left_by/right_by and left_index/right_index to merge_asof()
The current `merge_asof()` has only one parameter for `by`, assuming that both DataFrames have the same column name. It does have `left_on` and `right_on`, so perhaps something similar for `by` would be helpful.

Also, there is no `left_index` or `right_index` parameter in `merge_asof()`. Adding these might be helpful too.

For example:

```
pd.merge_asof(df1, df2,
              left_index=True, right_on='timestamp',
              left_by='Ticker', right_by='ticker')
```
