Fix Header Tool not supporting line breaks

[mrspence]

What's the issue?

Currently the Header Tool does not support line breaks like other Tools do. Normally a user can press Shift + Enter which adds a line break. However at the moment this tool strips line breaks out.

This pull request fixes:

• #73 - Header not allowing <br> when shift+enter pressed - editor-js/header repo
• #1032 - show texts with line breaks, inside a block - editor-js repo

Solution

This pull request adds a config parameter allowLineBreaks: {Boolean} that allows the developer to enable line breaks should they need.

config: {
    allowLineBreaks: true, // default: false
}

By default, line breaks are not enabled, as per the current status quo.