Alignment buttons act on non-selected paragraphs

[jameshoward]

Trumbowyg version: 2.5.1
Browser: Chrome 58.0.3029.81 on OSX

Steps to reproduce:
(Note: This can be done on the demo editor on https://alex-d.github.io/Trumbowyg/)

1. Enter 3 paragraphs of text (Or paste into the code editor: <p>Line 1<p><p>Line 2</p><p>Line 3</p>)
2. Select line 2 and line 3.
3. Click the align center button.
4. Click the align right button.

Expected: Line 1 remains left aligned, 2 & 3 are centered and then aligned right.
Actual: Line 1 remains left aligned initially, while 2 & 3 are centered but then also aligns right when the align right button is clicked.

[Alex-D]

Should be fixed in future v3 (See #875 for more details).

I close this to keep issues clean since this issue is old with no activity and no trivial fix.