Fread options not extended to excel files

- Did you find a bug in datatable, or maybe the bug found you?
I cant select columns or limit number of rows read when working on an excel file

- How to reproduce the bug?
```python
import urllib

url = 'https://github.com/samukweku/Extracting-Data-from-Excel-with-Python/blob/master/names.xlsx?raw=true'

filename = 'test.xlsx'

urllib.request.urlretrieve(url, filename)

fread("test.xlsx/naija")
Name Age Height
0 Tolu 24 2
1 Chukwuka 50 1.8
2 Ogor 15 1.5

# column selection fails
fread('test.xlsx/naija', columns={"Name"})

# nrows fails as well
fread("test.xlsx/naija", max_nrows=2)
```

- What was the expected behavior?

```python
# column selection :
fread('test.xlsx/naija', columns={"Name"})
Name
0 Tolu
1 Chukwuka
2 Ogor

# row selection :
fread("test.xlsx/naija", max_nrows=2)
Name Age Height
0 Tolu 24 2
1 Chukwuka 50 1.8

```
- Your environment?
```python
dt.__version__
'0.11.0a0+build.1600188275.sam'

sys.version
'3.8.5 | packaged by conda-forge | (default, Aug 29 2020, 01:22:49) \n[GCC 7.5.0]'

operating system
'Linux-5.4.0-7642-generic-x86_64-with-glibc2.10'
```