Added UI customisations

[ClaudioTSM]

Hello to everybody.

In this PR I have:

• added all the customisation proposed and never merged of PR #16
• implemented the feedback of PR #16 (mainly improving names)
• added the lineHeight optional property (with rounded corners)
• added the handleDiameter optional property
• added the selectedHandleDiameterMultiplier optional property
• added the tintColorBetweenHandles optional property
• updated the demo
• updated the documentation

I am sure this customisation will be useful to other devs, so I hope it can be merged. I will be waiting for your feedback.

Thanks!

[TomThorpe]

Hi,

Thanks a lot for this! I really appreciate it. I will try to check it as soon as possible.

Tom

[TomThorpe]

This is excellent - thanks very much for your contributions! Sorry for taking so long to review.

[ClaudioTSM]

Thanks!