Added support for the center alignment of the tabs while scrolling

Thanks a lot for your nice effort.

Today while using the library I wanted to make the selected tabs and indicator center aligned but it was quite difficult to do this with just ofsetting the scroll value.

So,  I Added the functionality to make the indicator and the active tab center aligned. While scrolling offset is very useful this will add more freedom if anyone wants the selected tab to be exactly located at the center of the screen.

by just changing the value of tabCenterAligned variable to true will make the selected tabs center aligned. :)

Again, thanks a lot for all the you've put into making this library. It really helped me a lot. :smile: