Added a fixOverflow boolean option to enable/disable body overflow-x handling.

I have a situation where the body has overflow-x: auto, and the content is wide enough to trigger a horizontal scrollbar. When triggeing a tooltip, the overflow is modified and the scrollbar dissapears, only to appear back when the tooltip is hidden. This produces unwanted visual effects (stuff that is sticky to the bottom of the page is moved down then up).

Also, the overflow-x property on the body is set through a css class applied to it. When trying to change tha body's class to some class that has a different overflow-x value, the inline style that was set by Tooltipster when the tooltip was hidden - overrides it.

A quickfix would be to add !important to my body class css declarations; however, this PR exposes a boolean option for enabling/disabling the overflow-x handling in Tooltipster. For BC, it's default value is true.