fix: value rounding issue in some cases

Was present with e.g. `<input type="range" min="0.1" max="3" step="0.1"
value="0.5" data-rangeslider>`
The value `1.7` returned `1.7000000000000002` and
`1.9` was `1.8999999999999997`.

* add dynamic toFixed digit based on step length to format the value

closes #103
example: add more floating point cases