Huge html with df.style.render to due css duplications
When creating html with `df.style` e.g.
```python
df.style.apply(color_f, axis=1).render()
```
pandas assigns to each html cell of the table a unique css class. This means huge html are created, while it would be possible to use the same class for cell with the same style.

<details>

------------------
commit: None
python: 2.7.14.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.14-300.fc27.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: None.None

pandas: 0.22.0
pytest: 3.1.3
pip: 9.0.1
setuptools: 36.0.1
Cython: 0.27.3
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.4.1
sphinx: None
patsy: 0.4.1
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.11
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
