filter_slider(min = NA) now gives a single handled slider

[cpsievert]

Closes #7

[wangchen46]

Hi @cpsievert, thanks for the update. I would like to check if this commit is planned to merge into main branch. My current project using filter_slider would like to implement both double range and single handler slider. If this commit will be included in package, that will be great to get more flexibility!