fix(placeholder): Use '__has-no-content' class name to display placeholder text (#461)
fix(placeholder): Use '__has-no-content' class name to display placeholder text (#461)

This causes the placeholder to appear even when the editor div is not
wholly devoid of elements.

Possible BREAKING CHANGE: The placeholder text is now absolutely
positioned (at `top: 0`) of the editor div. If a user has applied their
own padding to the editor div the placeholder may not show up in the
correct location. The solution is to add a CSS rule that overrides the
`top` to match the value of the `padding-top`:
```
.__mobiledoc-editor.__has-no-content:after {
  top: <value of padding-top>;
}
```

Also: The editor now has a `min-height: 1em;`
to ensure that the placeholder has a space in which to be displayed.

Fixes #407 #171
bustle/mobiledoc-kit,Hstry/mobiledoc-kit,mixonic/content-kit-editor,YoranBrondsema/mobiledoc-kit,bustlelabs/content-kit-editor,YoranBrondsema/mobiledoc-kit,bustlelabs/mobiledoc-kit,Hstry/mobiledoc-kit,mixonic/content-kit-editor,YoranBrondsema/mobiledoc-kit,bustle/mobiledoc-kit,bustlelabs/mobiledoc-kit,bustle/mobiledoc-kit,bustle/mobiledoc-kit,Hstry/mobiledoc-kit,bustlelabs/content-kit-editor,bustlelabs/mobiledoc-kit,mixonic/content-kit-editor,bustlelabs/content-kit-editor