Add decimal option to read_excel
The `pd.read_csv()` function has a `decimal` option, which allows it to parse European dates very easily:

```
pd.read_csv("afile.csv", decimal=',')
```

This option is not present in `pd.read_excel()`, but it could also be very useful there.

It would be even more useful if this option was a list of characters to interpret as decimal delimiters, instead of a single character.
