Tiny-slider: Place navigation controls at the bottom

[frankackermann]

__Issue description__: I have seen that it has been decided to put all control and navigation elements on top of the slider, due to accessibility (#4). This is totally fine, but in my case it is needed to have the navigation underneath. Since I could not find any option to place navigation controls at the bottom, I highly recommend to add an option for the position. This would make the slider more flexible without using the workaround with navContainer where one need to know how many elements are in the slider during markup generation. Also styling this additonal container causes more CSS code.

__Demo link/slider setting__:

slider = tns({
    container: '.container-class',
    controls: false,
    items: 1,
    nav: true,
    navPos: 'bottom' // not supported yet
});

_Tiny-slider version_: 2.8.7

[ganlanyuan]

added options: navPosition, controlsPosition and autoplayPosition in this commit.
I will update the document later

[ganlanyuan]

Document updated

[ciampam9]

@ganlanyuan 👋 when could we expect a release with this functionality?

[ganlanyuan]

released 2.8.8

[frankackermann]

Very useful. Thanks a lot!