ENH: Styler accepts css-strings input args as well as list of (attr,value) pairs
#### Is your feature request related to a problem?

Currently the `styler` has the ability to `set_table_styles` provided they are characterised with a css-selector and list of attribute value pairs, e.g.

```
styler.set_table_styles([{'selector': 'th', 'props': [('color', 'red'), ('font-size', '1em'), ('font-weight',  'bold')]}])
```

Even the legacy docs describe this as a bit of a pain, and for someone used to css, they will be typically looking for a css-style solution:

```
styler.set_table_styles([{'selector': 'th', 'props': 'color:red;font-size:1em;font-weight:bold;'}])
```

#### Describe the solution you'd like

optionally use the input argument valid CSS format above.

#### API breaking implications

none if backwards compatible.


