[Enhancement Request] Styler support for tooltips.
#### Problem description
It would be nice if pandas stlying allowed you to set a tooltip for table cells, in the same way that it allows you to set the CSS style using apply, applymap, etc.

Currently it is possible to create a hacky workaround using css-only tooltips and set_table_styles, but it adds substantial complexity to achieve something that is conceptually very simple.

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
pd.show_versions()


INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 36.0.1
Cython: 0.28.3
numpy: 1.14.3
scipy: 0.19.1
statsmodels: 0.8.0
xarray: None
IPython: 5.1.0
sphinx: None
patsy: 0.4.1+dev
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.2
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.1.0
xlwt: 0.7.5
xlsxwriter: None
lxml: 4.0.0
bs4: 4.6.0
html5lib: 0.999999999
httplib2: 0.9.2
apiclient: 1.6.5
sqlalchemy: 1.1.3
pymysql: 0.6.1.None
psycopg2: None
jinja2: 2.8
boto: 2.48.0
pandas_datareader: 0.2.0

</details>
