New: Modifier to add whitespace between example components (styleguid…

…ist#1456, fixes styleguidist#841)

Let users to be able to add padding between example components without
adding manual styling to their example code. This adds the `padded` modifier option
to the Playground that adds padding between each example component in that block.

```jsx padded
<Button>Push Me</Button>
<Button>Click Me</Button>
<Button>Tap Me</Button>
```

Fix: Allow isolated components with duplicate names (fixes styleguidi…

…st#1454)

If you have more than one component with the same name, the isolated mode will incorrectly show all of the components of the same name (instead of just the one you clicked on).

This fix solves the issue by prepending the existing hash path from the page the user came from. By prepending this hash path, Styleguidist correctly scopes the isolated example to just the desired component.

Changelog: 🚀

Mostly code cleanup (use official React Context API, remove deprecated React APIs and patterns, etc.) and dependencies updates.

## Breaking changes

* Node 8.9 is the lowest supported version
* React 16.8 is the lowest supported version

Docs: Add a screenshot

Fix: Update react-dev-utils to ^9.0.3 (styleguidist#1422)

Fix: Don't show empty sidebar (styleguidist#1423)

Fix: Single sections can contain only sections (styleguidist#1414)

If a section contains only one subsection, Styleguidist will try to optimize. It will make the section disappear and show its components instead. If this section only contain sections, no components, it should optimize to sections and not to components.

Fix: Highlighting currently selected page on sidebar broken with sect…

…ions (styleguidist#1405)

Fixes styleguidist#1375

Chore: Upgrade react-icons to 3.7.0 (styleguidist#1406)

Fix: Fix incorrect component name capitalization (styleguidist#1388)

Fixes styleguidist#1381

It is happening in a case when react-docgen (any propsParser) is failed and we are trying to guess displayName based on file path. We capitalized every case in the string instead of just words separated by `-` started with small letter. So, by mistake `ButtonTS` in file name becomes `ButtonTs` display name for component when we expect `ButtonTS` Use [startCase]:(https://lodash.com/docs/4.17.11#startCase) and then remove spaces which covers most our cases likemy-buttonTS => MyButtonTS