Make centered buttons have equal margin

[james-wasson]

This is a bugfix.

Proposed solution

Issue

When you have long buttons within .buttons.is-centered and a button moves to a lower line the alignment of each button is off.
Same issue with .buttons.is-right

Solution

Add equal margin to both sides of the button.

Tradeoffs

We are using button margin value of 0.5rem (0.25rem each side for .is-centered) that value could change so it might be beneficial to use a variable here instead.

Testing Done

I have some js-fiddles with my tests:

• Fiddle before fix: https://jsfiddle.net/james_wasson/a9nmyzcg/
• Fiddle after fix: https://jsfiddle.net/james_wasson/9kfnz3b0/

shrink the screen to see the broken buttons under

• Test Buttons Group -> Text Case 1 -> Centered
• Test Buttons Is-Right -> Any test case

Before:

After:

[james-wasson]

Updated pull request with new information about .is-right and .is-centered.has-addons

[seb3s]

I've upgraded from previous version 0.6.0. This update unfortunately breaks buttons right alignement with forms for normal size buttons (which is the common case).