pd.read_clipboard() from excel now gives warning
#### Code Sample, a copy-pastable example if possible

```python
"""
Copy below from excel
foo	bar
123	4546464
123	462462
23	26456246
123	2346246
"""
import pandas as pd
pd.read_clipboard()

# get the following error
"""
C:\...\lib\site-packages\pandas\io\clipboards.py:63: ParserWarning: Falling back to the 'python' engine because the 'c' engine does not support regex separators (separators > 1 char and different from '\s+' are interpreted as regex); you can avoid this warning by specifying engine='python'.
  return read_table(StringIO(text), sep=sep, **kwargs)
"""
```
#### Problem description
[this recent commit](https://github.com/pandas-dev/pandas/commit/dc45fbafef172e357cb5decdeab22de67160f5b7#diff-3f25860d9237143c1952a1f93c3aae18L102) makes `read_table` think the separator is more than 1 characters long. `parser.py` in function `_clean_options`

When changing back to

`sep = '\t'`

it works without warning. But maybe it breaks something else?

#### Expected Output
No warning, and being able to use engine 'c'

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 61 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.23.0
pytest: 3.5.1
pip: 18.0
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.1.0
pyarrow: 0.8.0
xarray: None
IPython: 6.4.0
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
