Add method to change block type

[kyselberg]

I found it very useful to use this function to set header tags (e.g. h1, h2, h3).

this.editor.changeBlockType('h1');
this.editor.changeBlockType('h2');
this.editor.changeBlockType('h3');

To be clear, it's your @neilj example from this PR.

If this type of method doesn't fit to the Squire's philosophy or plans right now, then could we expose couple "private" methods so we can build more upon the Squire (the best open-source editor out there imho) and not patch anything.

[legendetm]

This would be very helpful. With Squire 1.x it was possible to extend Squire, but with 2.x rewrite the internal method are not accessible anymore (not exported) and this makes extending very complicated (e.g. getBlockWalker, empty). Also the source is not included in package.