Disable useless nav buttons

[yucatio]

Disable useless button

• disable page-back/page-next button when it is in the first/last page
• disable 'now' button when it has no .today or can't scroll
• disable scroll buttons when it can't scroll or start/end of range
• disable zoom-in/out button when its scale is min/max

these feature require #196

[yucatio]

This change has bug reported by https://github.com/yucatio/yucatio.github.io/issues/1 I'll tell you when I fixed.

[yucatio]

Fixed bug