Add option for inline CSS styling
#### Code Sample, a copy-pastable example if possible

Taken from the [pandas docs](https://pandas.pydata.org/pandas-docs/stable/style.html):
```python
>>> df.style.highlight_null().render().split('\n')[:20]

['<style  type="text/css" >',
 '    #T_70282eae_d6e2_11e8_b967_f218980ce499row0_col2 {',
 '            background-color:  red;',
 '        }</style>  ',
 '<table id="T_70282eae_d6e2_11e8_b967_f218980ce499" > ',
 '<thead>    <tr> ',
 '        <th class="blank level0" ></th> ',
 '        <th class="col_heading level0 col0" >A</th> ',
 '        <th class="col_heading level0 col1" >B</th> ',
 '        <th class="col_heading level0 col2" >C</th> ',
 '        <th class="col_heading level0 col3" >D</th> ',
 '        <th class="col_heading level0 col4" >E</th> ',
 '    </tr></thead> ',
 '<tbody>    <tr> ',
 '        <th id="T_70282eae_d6e2_11e8_b967_f218980ce499level0_row0" class="row_heading level0 row0" >0</th> ',
 '        <td id="T_70282eae_d6e2_11e8_b967_f218980ce499row0_col0" class="data row0 col0" >1</td> ',
 '        <td id="T_70282eae_d6e2_11e8_b967_f218980ce499row0_col1" class="data row0 col1" >1.32921</td> ',
 '        <td id="T_70282eae_d6e2_11e8_b967_f218980ce499row0_col2" class="data row0 col2" >nan</td> ',
 '        <td id="T_70282eae_d6e2_11e8_b967_f218980ce499row0_col3" class="data row0 col3" >-0.31628</td> ',
 '        <td id="T_70282eae_d6e2_11e8_b967_f218980ce499row0_col4" class="data row0 col4" >-0.99081</td> ']
```
#### Problem description

In some circumstances (e.g. GMail), only inline CSS styling is supported. In these situations, it would be helpful to have an option to use inline CSS styling instead.

#### Expected Output
```python
>>> df.style.highlight_null().render(inline=True).split('\n')[:16]

['<table> ',
 '<thead>    <tr> ',
 '        <th></th> ',
 '        <th>A</th> ',
 '        <th>B</th> ',
 '        <th>C</th> ',
 '        <th>D</th> ',
 '        <th>E</th> ',
 '    </tr></thead> ',
 '<tbody>    <tr> ',
 '        <th>0</th> ',
 '        <td>1</td> ',
 '        <td>1.32921</td> ',
 '        <td style="background-color:red;">nan</td> ',
 '        <td>-0.31628</td> ',
 '        <td>-0.99081</td> ']
```
