Change visibility of Readability::loadHtml() and prevent DOM reset in init()

[Kdecherf]

These changes are required for j0k3r/graby#392

@j0k3r @jtojnar commit 8f8fb2c introduces a slightly changed behavior on init().

With this PR, calling twice init() does not reset $dom anymore.
I don't know if you want to consider this as a breaking change.

[jtojnar]

I think this should be fine. Nobody should be re-initing (re-executing really) a Readability instance. They should just make a new instance.

Do we know why it was moved in 66215a6? Could we perhaps just move it back? Though that might be a larger BC break than this.

Or alternately, move the HTML loading into a separate class and use it here and in Graby::getSinglePage(). Though that will not work for Graby\Extractor\ContentExtractor.

[j0k3r]

Do we know why it was moved in 66215a6?

Sadly, can't remember..

[Kdecherf]

Could we perhaps just move it back? Though that might be a larger BC break than this.

Yeah, moving back loadHtml() to __construct() may be a BC for those already using php-readability 2.x.
Also I triggered a different set of issues when trying this approach on my work for j0k3r/graby#392

I wonder if we shouldn't just go ahead and prevent init() to be called more than once

[Kdecherf]

The changes view is a bit cluttered by the move of the loadHtml() function by php-cs-fixer, here is a shorter view of the actual changes: https://github.com/j0k3r/php-readability/pull/113/changes/BASE..3f9f3dc2efacad494c142ed735eef7719f508d30

[j0k3r]

Good move 👍🏼